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NASA NTRS2020

SOFIA/HAWC plus Traces the Magnetic Fields in NGC 1068

We report the first detection of galactic spiral structure by means of thermal emission from magnetically aligned dust grains. Our 89μm polarimetric imaging of NGC 1068 with the High-resolution Airborne Wideband Camera/Polarimeter(HAWC+)on NASAs Stratospheric Observatory for Infrared Astronomy(SOFIA)also sheds light on magnetic field structure in the vicinity of the galaxyʼs inner-bar and active galactic nucleus(AGN).We find correlations between the 89μm magnetic field vectors and other tracers of spiral arms, and a symmetric polarization pattern as a function of the azimuthal angle arising from the projection and inclination of the disk field component in the plane of the sky. The observations can befit with a logarithmic spiral model with pitch angle of-+16.92.82.7and a disk inclination of 48°±2°. We infer that the bulk of the interstellar medium from which the polarized dust emission originates is threaded by a magnetic field that closely follows the spiral arms. Inside the central starburst disk(<1.6 kpc), the degree of polarization is found to be lower than for far-infrared sources in the Milky Way, and has minima at the locations of most intense star formation near the outer ends of the inner-bar. Inside the starburst ring, the field direction deviates from the model, becoming more radial along the leading edges of the inner-bar. The polarized flux and dust temperature peak∼3′′–6′′NE of the AGN at the location of a bow shock between the AGN outflow and the surrounding interstellar medium, but the AGN itself is weakly polarized(<1%)at both 53 and 89μm.

Active galactic nuclei↗

NASA NTRS1999

Saturn's Ring: Pre-Cassini Status and Mission Goals

In November 1980, and again in August 1981, identical Voyager spacecraft flew through the Saturn system, changing forever the way we think about planetary rings. Although Saturn's rings had been the only known ring system for three centuries, a ring system around Uranus had been discovered by stellar occultations from Earth in 1977, and the nearly transparent ring of Jupiter was imaged by Voyager in 1979 (the presence of material there had been inferred from charged particle experiments on Pioneer 10 and 11 several years earlier). While Saturn had thus temporarily lost its uniqueness as having the only ring system, with Voyager it handily recaptured the role of having the most fascinating one. The Voyager breakthroughs included spiral density and bending waves such as cause galactic structure; ubiquitous fine-scale radial 'irregular' structure, with the appearance of record-grooves; regional and local variations in particle color; complex, azimuthally variable ring structure; empty gaps in the rings, some containing very regular, sharp-edged, elliptical rings and one containing both a small moonlet and incomplete arcs of dusty material; and shadowy 'spokes' that flicker across the main rings. One of the paradigm shifts of this period was the realization that many aspects of planetary rings, and even the ring systems themselves, could be 'recent' on geological timescales. These early results are reviewed and summarized in the Arizona Space Science series volumes 'Saturn'. (An excellent review of ring dynamics at a formative stage is by Goldreich and Tremaine.) From the mid 1980's to the time of this writing, progress has been steady, while at a less heady pace, and some of the novel ring properties revealed by Voyager 1 and 2 are beginning to be better understood. It is clearly impossible to cite, much less review, every advance over the last decade; however, below we summarize the main advances in understanding of Saturn's rings since the mid 1980's, in the context of the Cassini Science Objectives.

Cuzzi, Jeff N.↗

NASA NTRS1986

Measuring interstellar dust grains from the haloes of binary X-ray sources

A method for determining the location, size distribution, and composition of interstellar dust grains from the haloes of binary X-ray sources is described. The method is based on the time dependence of the halo X-ray emission during an eclipse of the point source. The distance of the dust cloud is inferred directly from observations of the growth of the dark circle around the eclipsed point source. Grain size distribution in the interstellar dust clouds is derived from the halo brightness profile at a given photon energy. The composition of the grains is obtained by observing the spectral features in the total optical depth due to X-ray scattering near the photoionization thresholds of the elements in the grains. The haloes become dim just below the ionization threshold, and sharply increase in function brightness as the photon energy increases beyond the absorption edge of the relevant atom. The method makes it possible to observe and analyze the halo emission during eclipse even without imaging the source. It is suggested that haloes probably account for the soft X-rays observed during eclipses and dips observed in sources including 4U1915-05 and X1755-338.

Xu, Y.↗

NASA NTRS2002

The X-ray Reflectors in the Nucleus of the Seyfert Galaxy NGC 1068

Based on observations of the Seyfert nucleus in NGC 1068 with ASCA, RXTE and BeppoSAX, we report the discovery of a flare (increase in flux by a factor of approximately 1.6) in the 6.7 keV Fe K line component between observations obtained four months apart, with no significant change in the other (6.21, 6.4, and 6.97 keV) Fe Kalpha line components. During this time, the continuum flux decreased by approximately 20%. The RXTE spectrum requires an Fe K absorption edge near 8.6 keV (Fe XXIII- XXV). The spectral data indicate that the 2-10 keV continuum emission is dominated (approximately 2/3 of the luminosity) by reflection from a previously unidentified region of warm, ionized gas located approximately or less than 0.2 pc from the AGN. The remaining approximately 1/3 of the observed X-ray emission is reflected from optically thick, neutral gas. The coronal gas in the inner Narrow-Line Region (NLR) and/or the cold gas at the inner surface of the obscuring 'torus' are possible cold reflectors. The inferred properties of the warm reflector are: size (diameter) approximately or less than 0.2 pc, gas density n approximately or greater than 10(exp 5.5)/cu cm, ionization parameter xi is approximately 10(exp 3.5) erg cm s(exp -1), and covering fraction 0.003 (L(sub 0)/ 10(exp 43.5) erg s(exp -1)(exp -1) less than (omega/4pi) less than 0.024 (L(sub 0)/ 10(exp 43.5) erg s(exp -1) (exp -1) where L(sub 0) is the intrinsic 2-10 keV X-ray luminosity of the AGN. We suggest that the warm reflector gas is the source of the (variable) 6.7 keV Fe line emission, and the 6.97 keV Fe line emission. The 6.7 keV line flare is assumed to be due to an increase in the emissivity of the warm reflector gas from a decrease (by 20-30%) in L(sub 0). The properties of the warm reflector are most consistent with an intrinsically X-ray weak AGN with L(sub 0) approximately equals 10(exp 43.0) erg s(exp -1). The optical and UV emission that scatters from the warm reflector into our line of sight is required to suffer strong extinction, which can be reconciled if the line-of-sight skims the outer surface of the torus. Thermal bremsstrahlung radio emission from the warm reflector may be detectable in VLBA radio maps of the NGC 1068 nucleus.

Colbert, Edward J. M.↗

NASA NTRS2018

Hi-C2.1 Observations of Solar Jetlets at Edges of Network Lanes

We present high resolution (0.2”) high cadence (5s) extreme ultraviolet (EUV) observations of small-scale jetlet-like features and spicule-like features observed with NASA’s High-resolution Coronal Imager2.1 (Hi-C) during its 5‐min observing span. We investigate the magnetic setting of 4 on-disk jetlets and 2 on-disk spicules by using high resolution 172A images from Hi-‐C and EUV images from Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and line-of-sight magnetograms from SDO/Helioseismic and Magnetic Imager (HMI). The jetlets and spicules are at edges of magnetic network lanes. From magnetograms co-aligned with the Hi-C and AIA images, we find that the jetlets stem from sites of likely flux cancelation between merging majority-polarity and weaker minority-polarity flux, but in contrast to larger jetlets observed by IRIS some do not show obvious enhanced brightenings at their base. Based on the similarity of these observations of approx. 4 obvious Hi-C small jetlets with our previous observations of 10 IRIS larger jetlets and approx. 30 coronal jets in quiet regions and coronal holes, we infer that flux cancelation is probably the essential process in the buildup and triggering of jetlets. Our observations suggest that network jetlet eruptions, large and small, are small-scale analogs of both larger-scale coronal jet eruptions and the still-larger-scale eruptions that makemajor CMEs. For simplicitywe will use termjetlets for jetlet-like features and spicules for spicule-like features.

Panesar, Navdeep K.↗

NASA NTRS1982

High velocity gas outflow from the nova-like variable LSI 55 deg -8

Low and high dispersion International Ultraviolet Explorer observations and measurements of the optical brightness of LSI 55-8 taken on 5 December, 1981 are analyzed. The presence of the C IV resonance doublet showing a variable P Cygni type of profile with a blue shifted absorption component along with the blue shifted absorption features of the high temperature ions indicate the existence of outflowing hot plasma (i.e., a wind). An upper limit of the mass outflow rate was established from the shot wavelength edge of the C IV absorption. The normalized C IV profile is shown o be variable with the largest changes occurring in the relative strength and shape of the emission component. These changes appear to be systematic, and may be associated with the changing projection of the outflowing hot plasma against the disk as a function of orbital motion. LSI 55-8 demonstrates most of the spectral and photometric characteristics exhibited by the nova like, UX UMa type variables, as well as with the dwarf novae in outburst or standstill. The high accretion rate inferred from fitting the Williams theoretical disk model fluxes to IUE data support the hypothesis that LSI 55-8 and related nova like systems are in a state of prolonged outburst.

Guinan, E. F.↗

NASA NTRS1992

Glacio-isostatic crustal movements caused by historical volume change of the Vatnajokull ice cap, Iceland

Measurements of the lake level of Lake Langisjor at the SW edge of the Vatnajokull ice cap indicate a tilt of 0.26 +/- 0.06 microrad/yr away from the ice cap in the years of 1959-1991. The tilt is too large to be explained as an elastic Earth response to ice retreat this century, or to be caused by change in the gravitational pull of the ice cap, but it can be explained by sub-lithospheric viscous adjustment. Regional subsidence in historical times in SE Iceland can similarly be attributed to viscous adjustment resulting from the increased load of Vatnajokull during the Little Ice Age. The inferred sublithospheric viscosity is 1 x 10 exp 18 - 5 x 10 exp 19 Pa s.

Sigmundsson, Freysteinn↗

NASA NTRS1993

Pigmenting agents in Martian soils: inferences from spectral, Mossbauer, and magnetic properties of nanophase and other iron oxides in Hawaiian palagonitic soil PN-9

We have examined a Hawaiian palagonitic tephra sample (PN-9) that has spectroscopic similarities to Martian bright regions using a number of analytical techniques, including Mossbauer and reflectance spectroscopy, X-ray diffraction, instrumental neutron activation analysis, electron probe microanalysis, transmission electron microscopy, and dithionite-citrate-bicarbonate extraction. Chemically, PN-9 has a Hawaiitic composition with alkali (and presumably silica) loss resulting from leaching by meteoric water during palagonitization; no Ce anomaly is present in the REE pattern. Mineralogically, our results show that nanophase ferric oxide (np-Ox) particles (either nanophase hematite (np-Hm) or a mixture of ferrihydrite and np-Hm) are responsible for the distinctive ferric doublet and visible-wavelength ferric absorption edge observed in Mossbauer and reflectivity spectra, respectively, for this and other spectrally similar palagonitic samples. The np-Ox particles appear to be imbedded in a hydrated aluminosilicate matrix material; no evidence was found for phyllosilicates. Other iron-bearing phases observed are titanomagnetite, which accounts for the magnetic nature of the sample; olivine; pyroxene; and glass. By analogy, np-Ox is likely the primary pigmenting agent of the bright soils and dust of Mars.

Minerals/analysis↗

NASA NTRS1999

Spectral Behavior of Hematite at Visible/Near Infrared and Midinfrared Wavelengths

The oxide mineral hematite alpha-Fe2O3) is present on Mars as evidenced by spectroscopy using visible/near infrared (VNIR) and midinfrared (MIR) wavelengths. The physical nature of this surficial hematite varies from fine-grained, crystalline red hematite (red-Hm) in certain martian bright regions, to coarser-grained, crystalline gray hematite (gray-Hm) in the Sinus Meridiani equatorial site discovered in the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data. The VNIR spectra require that the red hematite is present in amounts subordinate to a second ferric pigment, which accounts for the general shape and position of the martian ferric absorption edge. By analogy with palagonitic tephra and synthetic samples, the pigment is a nanophase ferric oxide, possibly nanophase hematite (np-Hm). The purpose of this abstract is to document the physicospectral properties of hematite in the VNIR (0.35 to 2.2 microns) and MIR (5 to 25 microns) with respect to the terms "nanophase" and "crystalline", "red" and "gray", and "fine-grained" and "coarse-grained". We will show that different "types" of hematite (no-, red-, and gray-Hm) have different spectral responses in the VNIR and MIR regions and that these differences are important for inferring the types of hematite and their relative proportions from remotely sensed spectra of Mars. Because hematite is an alteration product that forms through many different geologic processes, determination of the type of hematite provides important clues about the style and diversity of martian weathering processes.

Lane, Melissa D.↗

NASA NTRS1999

Spectral Behavior of Hematite at Visible/Near Infrared and Midinfrared Wavelengths

The oxide mineral hematite alpha-Fe2O3) is present on Mars as evidenced by spectroscopy using visible/near infrared (VNIR) and midinfrared (MIR) wavelengths. The physical nature of this surficial hematite varies from fine-grained, crystalline red hematite (red-Hm) in certain martian bright regions, to coarser-grained, crystalline gray hematite (gray-Hm) in the Sinus Meridiani equatorial site discovered in the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data. The VNIR spectra require that the red hematite is present in amounts subordinate to a second ferric pigment, which accounts for the general shape and position of the martian ferric absorption edge. By analogy with palagonitic tephra and synthetic samples, the pigment is a nanophase ferric oxide, possibly nanophase hematite (np-Hm). The purpose of this abstract is to document the physicospectral properties of hematite in the VNIR (0.35 to 2.2 mm) and MIR (5 to 25 mm) with respect to the terms "nanophase" and "crystalline", "red" and "gray", and "fine-grained" and "coarse-grained". We will show that different "types" of hematite (np-, red-, and gray-Hm) have different spectral responses in the VNIR and MIR regions and that these differences are important for inferring the types of hematite and their relative proportions from remotely sensed spectra of Mars. Because hematite is an alteration product that forms through many different geologic processes, determination of the type of hematite provides important clues about the style and diversity of martian weathering processes. Additional information is contained in the original extended abstract.

Lane, M. D.↗

NASA NTRS1989

Plasma pressure in the environment of Jupiter, inferred from Voyager 1 magnetometer observations

A model combining the internal magnetic field with a self-consistent model of the Jovian magnetodisc was fitted to the Voyager 1 Jovian magnetic field data by means of a generalized inverse technique. The model parameters included the internal field spherical harmonic coefficients as well as with parameters describing the plasma distribution in the magnetosphere. Assuming that the pressure in the middle and outer magnetosphere is related to the unit flux tube volume V through PV exp gamma = const, the model fit yielded a value of 0.88 for gamma. If the hot (30 keV) plasma is transported adiabatically inward under the interchange instability triggered by centrifugal force of the heavy torus ions, losses are not sufficient to account for such a low value of gamma beyond L = 10. Closer to the planet, as the outer edge of the Io plasma torus is approached, PV exp gamma is found to decrease inward, as expected from the particle measurements, which identified an inner boundary of the particle fluxes in that region. With the present stage of the development of magnetodisc models, secular variations of the internal field still remain difficult to estimate.

Caudal, G.↗

NASA NTRS2009

Detection of Subsurface Material Separation in Shuttle Orbiter Slip-Side Joggle Region of the Wing Leading Edge using Infrared Imaging Data from Arc Jet Tests

The objective of the present study was to determine whether infrared imaging (IR) surface temperature data obtained during arc-jet tests of Space Shuttle Orbiter s reinforced carbon-carbon (RCC) wing leading edge panel slip-side joggle region could be used to detect presence of subsurface material separation, and if so, to determine when separation occurs during the simulated entry profile. Recent thermostructural studies have indicated thermally induced interlaminar normal stress concentrations at the substrate/coating interface in the curved joggle region can result in local subsurface material separation, with the separation predicted to occur during approach to peak heating during reentry. The present study was an attempt to determine experimentally when subsurface material separations occur. A simplified thermal model of a flat RCC panel with subsurface material separation was developed and used to infer general surface temperature trends due to the presence of subsurface material separation. IR data from previously conducted arc-jet tests on three test specimens were analyzed: one without subsurface material separation either pre or post test, one with pre test separation, and one with separation developing during test. The simplified thermal model trend predictions along with comparison of experimental IR data of the three test specimens were used to successfully infer material separation from the arc-jet test data. Furthermore, for the test specimen that had developed subsurface material separation during the arc-jet tests, the initiation of separation appeared to occur during the ramp up to the peak heating condition, where test specimen temperature went from 2500 to 2800 F.

Daryabeigi, Kamran↗

NASA NTRS1988

Geodetic Measurement of Deformation East of the San Andreas Fault in Central California

Triangulation and trilateration data from two geodetic networks located between the western edge of the Great Valley and the San Andreas fault have been used to calculate shear strain rates in the Diablo Range and to estimate the slip rate along the Calaveras and Paicines faults in Central California. Within the Diablo Range the average shear strain rate was determined for the time period between 1962 and 1982 to be 0.15 + or - 0.08 microrad/yr, with the orientation of the most compressive strain at N 16 deg E + or - 14 deg. The orientation of the principal compressive strain predicted from the azimuth of the major structures in the region is N 25 deg E. It is inferred that the measured strain is due to compression across the folds of this area: the average shear straining corresponds to a relative shortening rate of 4.5 + or - 2.4 mm/yr. From an examination of wellbore breakout orientations and the azimuths of P-axes from earthquake focal mechanisms the inferred orientation of maximum compressive stress was found to be similar to the direction of maximum compressive strain implied by the trend of local fold structures. Results do not support the hypothesis of uniform fault-normal compression within the Coast Ranges. From trilateration measurements made between 1972 and 1987 on lines that are within 10 km of the San Andreas fault, a slip rate of 10 to 12 mm/yr was calculated for the Calaveras-Paicines fault south of Hollister. The slip rate of the Paicines fault decreases to 4 mm/yr near Bitter.

Sauber, Jeanne M.↗

NASA NTRS2006

Hubble Space Telescope NICMOS Polarization Measurements of OMC-1

We present 2 micrometer polarization measurements of positions in the BN region of the Orion Molecular Cloud (OMC-1) made with NICMOS Camera 2 (0.2" resolution) on Hubble Space Telescope. Our goals are to seek the sources of heating for IRc2, 3, 4, and 7, identify possible young stellar objects (YSOs), and characterize the grain alignment in the dust clouds along the lines-of-sight to the stars. Our results are as follows: BN is approximately 29% polarized by dichroic absorption and appears to be the illuminating source for most of the nebulosity to its north and up to approximately 5" to its south. Although the stars are probably all polarized by dichroic absorption, there are a number of compact, but non-point-source, objects that could be polarized by a combination of both dichroic absorption and local scattering of star light. We identify several candidate YSOs, including an approximately edge-on bipolar YSO 8.7" east of BN, and a deeply-embedded IRc7, all of which are obviously self-luminous at mid-infrared wavelengths and may be YSOs. None of these is a reflection nebula illuminated by a star located near radio source I, as was previously suggested. Other IRc sources are clearly reflection nebulae: IRc3 appears to be illuminated by IRc2-B or a combination of the IRc2 sources, and IRc4 and IRc5 appear to be illuminated by an unseen star in the vicinity of radio source I, or by Star n or IRc2-A. Trends in the magnetic field direction are inferred from the polarization of the 26 stars that are bright enough to be seen as NICMOS point sources. Their polarization ranges from N less than or equal to 1% (all stars with this low polarization are optically visible) to greater than 40%. The most polarized star has a polarization position angle different from its neighbors by approximately 40 degrees, but in agreement with the grain alignment inferred from millimeter polarization measurements of the cold dust cloud in the southern part of OMC-1. The polarization position angle of another highly-polarized, probable star also requires a grain alignment and magnetic field orientation substantially different from the general magnetic field orientation of OMC-1.

Simpson, Janet P.↗

NASA NTRS2002

Thermal and Nonthermal X-ray Emission from the Forward Shock in Tycho's Supernova Remnant

We present Chandra CCD images of Tycho's supernova remnant that delineate its outer shock, seen as a thin, smooth rim along the straight northeastern edge and most of the circular western half. The images also show that the Si and S ejecta are highly clumpy, and have reached the forward shock at numerous locations. Most of the X-ray spectra that we examine along the rim show line emission from Si and S, which in some cases must come from ejecta; the continuum is well represented by either thermal or nonthermal models. In the case that the continuum is assumed to be thermal, the temperatures at the rim are all similar at about 2 keV, and the ionization ages are very low because of the overall weakness of the line emission. Assuming shock velocities inferred from radio and X-ray expansion measurements, these temperatures are substantially below those expected for equilibration of the electron and ion temperatures; electron to mean temperature ratios of approximately less than 0.1 - 0.2 indicate at most modest collisionless heating of the electrons at the shock. The nonthermal contribution to these spectra may be important, however, and may account for as many as half of the counts in the 4-6 keV energy range, based on an extrapolation of the hard X-ray spectrum above 10 keV.

Hwang, Una↗

NASA NTRS1989

Albedo patterns on Triton

A model for seasonal taransport of volatiles based on Trafton (1984) is coupled to considerations of methane photochemistry to infer albedo patterns on Triton's surface at the time of the Voyager encounter. The relatively large southern subsolar latitude at present is predicted to cause the deposition of an extensive fresh frost layer covering approximately the northern half of the planet. Sublimation of Triton's south polar ice has been occurring since 1930, but has been preferentially occurring near the pole. The sublimation of ices in this region may be concentrating dark organic matter on the surface of the ice or exposing layers of this material which have built up in the ice over many seasonal cycles. The south polar cap may be distinctly darker near the pole than around the equatorward edge.

Stansberry, John A.↗

NASA NTRSDate not supplied

Location, location: deciphering the significance of in-situ hydrogen analyses of Martian meteorite phases

Understanding how inner planets acquired their volatiles such as hydrogen (H) is fundamental to constrain models of solar system formation [e.g. 1]. One avenue of estimating the H content and isotopic characteristics of differentiated planets is to analyze the samples we have from them as meteorites. Ideally, the H content and isotopic characteristic of the mantle sources of these igneous rocks should give insight into the volatile origin of each planetary body. The mantle H signatures can be estimated from that of the parent melt, which in turn may be derived from that of the first crystallized phases. However, we will illustrate the processes that can modify the H content and D/H ratios of pyroxene, olivine and feldspar in selected Martian meteorites relative to those of their mantle sources, with two key protocols for SIMS analysis. The first key is to put each analysis in textural context ("location, location"). In particular, of prime importance is whether an analysis is done at the center or the edge of a mineral grain, close or far from a shock disturbed area, and in a mineral crystallized at the beginning or late in the differentiation sequence. Accompanying the H analyses done by SIMS with major and trace element data at the same analysis locations allows to constrain the history of crystallization, cooling, alteration and shock of the meteorite. For example, volcanic degassing can be evidenced by decreasing water contents and increasing D/H ratios from core to edge of nakhlite pyroxenes [2]. Traverses of H analyses in pyroxene and maskelynite in shergottite LAR 06319 provide examples of H contents and D/H ratios modified during degassing following shock [3]. The second key is to assess if the area analyzed by SIMS has shock generated damage of the mineral structure, as will be shown in pyroxene and olivine from shergottite RBT 04262. Olivine in shergottites have too high water contents to be explained with an igneous origin and their D/H ratios are more consistent with terrestrial alteration, as observed before [4]. A review of all these processes shows how careful one has to be prior to using H measurements in meteorites to infer the origin and amount of water in differentiated planetary interiors.

A.H. Peslier↗

NASA NTRS2008

STEREO/SECCHI Stereoscopic Observations Constraining the Initiation of Polar Coronal Jets

We report on the first stereoscopic observations of polar coronal jets made by the EUVI/SECCHI imagers on board the twin STEREO spacecraft. The significantly separated viewpoints (approximately 11 degrees ) allowed us to infer the 3D dynamics and morphology of a well-defined EUV coronal jet for the first time. Triangulations of the jet's location in simultaneous image pairs led to the true 3D position and thereby its kinematics. Initially the jet ascends slowly at approximately equal to 10-20 kilometers per second and then, after an apparent 'jump' takes place, it accelerates impulsively to velocities exceeding 300 kilometers per second with accelerations exceeding the solar gravity. Helical structure is the most important geometrical feature of the jet which shows evidence of untwisting. The jet structure appears strikingly different from each of the two STEREO viewpoints: face-on in the one viewpoint and edge-on in the other. This provides conclusive evidence that the observed helical structure is real and is not resulting from possible projection effects of single viewpoint observations. The clear demonstration of twisted structure in polar jets compares favorably with synthetic images from a recent MHD simulation of jets invoking magnetic untwisting as their driving mechanism. Therefore, the latter can be considered as a viable mechanism for the initiation of polar jets.

Patsourakos, S.↗