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At least 451 records · Page 25
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NASA NTRS1993

Correlated variations of UV and radio emissions during an outstanding Jovian auroral event

An exceptional Jovian aurora was detected in the FUV on December 21, 1990, by means of Vilspa and Goddard Space Flight Center (GFSC) International Ultraviolet Explorer (IUE) observations. This event included intensification by a factor of three between December 20 and 21, leading to the brightest aurora identified in the IUE data analyzed, and, in the north, to a shift of the emission peak towards larger longitudes. The Jovian radio emission simultaneously recorded at decameter wavelengths in Nancay also exhibits significant changes, from a weak and short-duration emission on December 20 to a very intense one, lasting several hours, on December 21. Confirmation of this intense radio event is also found in the observations at the University of Florida on December 21. The emissions are identified as right-handed Io-independent 'A' (or 'non Io-A') components from the northern hemisphere. The radio source region deduced from the Nancay observations lies, for both days, close to the UV peak emission, exhibiting in particular a similar shift of the source region toward larger longitudes from one day to the next. A significant broadening of the radio source was also observed and it is shown that on both days, the extent of the radio source closely followed the longitude range for which the UV brightness exceeds a given threshold. The correlated variations, both in intensity and longitude, strongly suggest that a common cause triggered the variation of the UV and radio emissions during this exceptional event. On one hand, the variation of the UV aurora could possibly be interpreted according to the Prange and Elkhamsi (1991) model of diffuse multicomponent auroral precipitation (electron and ion): it would arise from an increase in the precipitation rate of ions together with an inward shift of their precipitation locus from L approximately equal 10 to L approximately equal 6. On the other hand, the analysis of Ulysses observations in the upstream solar wind suggests that a significant disturbance in the solar wind, involving the generation of an interplanetary shock and the presence of a CME have interacted with the Jovian magnetosphere at about the time of the auroral event. Both arguments suggest that we may have observed for the first time a magnetic storm-type interaction in an outer planet magnetosphere, affecting simultaneously several auroral processes. Conversely, the observed relationship between the level of UV auroral activity and the detection of decameter emission (DAM), if it were a typical feature, might argue in favour of a more direct and permanent association between the auroral processes leading to UV and radio aurorae, possibly related to 'discrete-arc'-like activity and electron precipitation.

Prange, R.↗
NASA NTRS1995

Heliospheric 2-3 kHz radio emissions and their relationship to large Forbush decreases

Two intense heliospheric 2-3 kHz radio emission events have been observed by Voyagers 1 and 2, the first in 1983-84 and the second in 1992-93. These radio emission events occurred about 400 days after large Forbush decreases in mid-1982 and mid-1991. Since Forbush decreases are indicative of a strong interplanetary shock propagating outward through the heliosphere, this temporal relationship provides strong evidence that the radio emissions are triggered by the interaction of a shock with one of the outer boundaries of the heliosphere. From the travel time and the known speed of the shock, the distance to the interaction region can be estimated and is well beyond 100 AU. At this great distance the plasma frequency at the terminal shock (100 to 200 Hz) is believed to be too small to explain the observed emission frequencies, which extend up to 3.6 kHz. For this reason, we have proposed that the interaction takes place at or near the heliopause, where remote sensing measurements show that the plasma frequency is in a suitable range (approximately 3 kHz) for explaining the radio emission. From the travel time and shock propagation speed, the radial distance to the heliopause has been calculated for various candidate solar events. After taking into account the likely deceleration of the shock, the heliopause is estimated to be in the range from about 110 to 160 AU.

Gurnett, D. A.↗
NASA NTRS1993

Solar cycle variation of interplanetary disturbances observed as Doppler scintillation transients

Interplanetary disturbances characterized by plasma that is more turbulence and/or moves faster than the background solar wind are readily defected as transients in Doppler scintillation measurements of the near-Sun solar wind. Systematic analysis of over 23,000 hours of Pioneer Venus Orbiter Doppler measurements obtained inside 0.5 AU during 1979-1987 have made it possible for the first time to investigate the frequency of occurrence of Doppler scintillation transients under solar minimum conditions and to determine its dependence on solar cycle. On the basis of a total of 142 transients, Doppler scintillation transient rates vary from a high of 0.22 in 1979 (one every 4.6 days) to a low of 0.077 transients/d in 1986 (one every 13 days), a decrease by almost a factor of 3 from solar maximum to solar minimum. This solar cycle variation, the strongest yet of any solar wind Doppler scintillation property, is highly correlated with both solar activity characterized by sunspot number and the coronal mass ejection rates deduced from Solswind and Solar Maximum Mission (SMM) coronagraph observations. These results indicate that coronal mass ejections and Doppler scintillation transients are closely related not just during solar maximum, as occasional individual comparisons have shown in the past, but throughout the entire solar cycle, and strengthen the notation that the Doppler scintillation and optical transients are different manifestations of the same physical phenomenon. The magnitudes of the transients, as described by the ratio of peak to pretransient scintillation levels (EF for enhancement factor), and their distribution iwth heliocentric distance also vary with solar cycle. While EF tends to diminish with increasing heliocentric distance during high solar activity, it is more evenly distributed during low solar activity. EF is also lower during solar minimum, as 13% of the transients during solar maximum have values exceeding 23, the highest EF observed during solar minimum. These results are consistent with the fact that occasional major fast-moving interplanetary shocks that are observed during solar maximum are very rate during solar minimum.

Woo, Richard↗
NASA NTRS1995

Mass ejections from the sun and their interplanetary counterparts

Since the first observations of solar mass ejection events in the early seventies from OSO 7 and Skylab a few thousand of these remarkable dynamic incidents have been observed by now, covering about two full solar activity cycles. The mass ejecta include mainly hot coronal plasma, plus cold prominence material in variable amounts. The ejecta are often recognised in the form of interplanetary plasma clouds detected in the distant solar wind by appropriately located spacecraft. Clouds which have been energetic enough to drive large scale interplanetary shock waves can be identified most readily, but clouds without associated shocks do also occur. The plasma clouds are characterized by a variety of signatures indicating that they actually originate from injections of different material into the ambient solar wind. Usually only a few of the signatures are found simultaneously. Apparently the bidirectional streaming of halo electrons is a most reliable criterion, indicating a magnetic bottle or plasmoid topology of the clouds. The discussion of the most recent discoveries in this context will show that quite a few crucial problems still remain to be addressed by the upcoming SOHO mission.

Schwenn, R.↗
NASA NTRS1995

Energetic particles and coronal mass ejections in the high latitude heliosphere: Ulysses-LET observations

The COSPIN Low Energy Telescope (LET) onboard the Ulysses spacecraft measures protons, alphas and heavier ions at energies of approximately 1 to 50 MeV/n. Ulysses measurements offer favorable opportunities to study the effects of solar activity in the out-of-ecliptic regions of the heliosphere. Using LET data, we have investigated the properties of transient energetic ions at high heliographic latitudes when Ulysses was permanently immersed in high speed solar wind and magnetically connected to the Sun on open magnetic field lines. Recurrent increases in the fluxes of energetic ions at high heliographic latitudes at frequencies related to the solar rotation period were found to occur in association with co-rotating interaction regions (CIRs). Here we investigate fluxes of energetic particles that showed no relationship to ClRs. From the investigation of plasma and magnetic held data it is found that all of the transient high latitude particle events were associated with the passage of a coronal mass ejection (CME) over Ulysses. Enhancements in particle fluxes several days prior to the arrival of a CME, but with a significant time delay with respect to the estimated CME-onset at the Sun, were most probably associated with interplanetary shocks driven by fast CMEs. These particle events exhibit unusually high rho/alpha-ratios and are not observed for CMEs not driving a shock. However, not all CMEs that passed Ulysses were associated with a particle event. We find evidence that at high solar latitudes, solar flare particles cannot reach Ulysses on open magnetic field lines, but can reach the spacecraft if particles are injected into magnetic flux-ropes (CMEs) at the Sun. These findings are supported by soft X-ray observations from the Japanese Yohkoh-satellite.

Bothmer, V.↗
NASA NTRS1995

Energetic particles as probes of solar wind disturbances

We have investigated the response of particles, in the energy range approximately 1-5000 MeV, to interplanetary shocks and coronal ejecta. Shocks can accelerate particles or cause decreases in particle densities. Ejecta cause decreases. Thus particle observations can provide information about solar wind disturbances. Of particular interest is that the boundaries of ejecta can often be more readily identified from approximately l GeV particle decrease observations than from most other phenomena associated with ejecta. We will discuss the properties of less energetic shocks and ejecta and compare them with those of the more energetic events which are normally discussed in the context of Forbush decreases, large proton events, etc. We use data from both Helios spacecraft and IMP 8 which allows some spatial variations to be studied.

Cane, H. V.↗
NASA NTRS1995

Recent observations and analyses of heliospheric 2-3 kHz radio emissions

Since the discovery of the 2-3 kHz heliospheric radio emissions in the Voyager data over twelve years ago, there have been two major events, the first in 1983-84 and the second in 1992-93, as well as several minor events. Strong evidence now exists that these radio emissions are generated by interactions that take place in the outer regions of the heliosphere, most likely in response to strong interplanetary shocks and associated disturbances propagating outward from the Sun. Just where these interactions take place is still a subject of controversy. The three main possibilities are at or near the termination shock, between the termination shock and the heliopause and at or near the heliopause. In this paper we will review the present state of knowledge concerning the 2-3 kHz radio emissions, including the most recent data from Voyagers 1 and 2. These observations will then be compared to models that have been proposed to explain the origin of the radio emissions. Depending on the model, various estimates can be made concerning the plasma densities in the region where the radio emission is produced, and the distance to the source. The constraints provided by these models on the size and structure of the heliosphere are discussed.

Gurnett, D. A.↗
NASA NTRS1995

Large-scale structure of solar wind as observed on the Prognoz 7 satellite

Properties of different solar wind streams depend on the large scale structure of coronal magnetic field and dynamical phenomena in the solar atmosphere. We present average values and distributions of MHD parameters (density, velocity, temperature, fluxes of mass, momentum and energy, ratio of thermal and magnetic pressures, as well as helium abundance) as observed on board the Prognoz 7 satellite in the different types of the solar wind streams connected with solar corona structure and phenomena: (1) heliospheric current sheet, (2) streams from coronal holes, (3) streams from coronal streamers, (4) plasma disturbed by interplanetary shocks, and (5) coronal mass ejections. As for quasistationary streams of solar wind, maximum mass flux is recorded in the streams emanating from the coronal streamers while maximum thermal and kinetic energy fluxes are observed in the streams from the coronal holes. The momentum fluxes are equal in both types of streams. Maximum ratio of thermal and magnetic pressures is observed in heliospheric current sheet. The maximum helium abundance is observed in coronal mass ejection, it is higher in streams from coronal holes than in streams from streamers, and its dependences on density and mass flux are different in different types of the streams. Dynamics of alpha-particle velocity and temperature relative to protons in different streams is discussed.

Yermolaev, Yu. I.↗
NASA NTRS1994

Modeling Ionospheric Convection During a Major Geomagnetic Storm on October 22-23, 1981

Following the passage of an interplanetary shock at approximately 0500 UT, a major geomagnetic storm developed on October 22-23, 1981. Numerous auroral substorms occurred during this storm leading to an AE index greater than 1000 nT. We have used the expanding/contracting polar cap (ECPC) model (Moses et al., 1989) and data from the Dynamics Explorer 2 spacecraft to study the ionospheric electric fields for 12 consecutive traversals of the polar regions. The ECPC model can determine the voltage drops across the dayside merging and nightside reconnection gaps. We determined the relationship of the AL index (i.e., the intensity of the westward electrojet) to the nightside reconnection potential drop. An excellent linear correlation was found between the nightside reconnection gap voltage drop and the AL index. These results show that the solar wind strongly drives the magnetosphere-ionosphere system throughout the geomagnetic storm. A substantial level of dayside merging seems to occur throughout the event. Nightside reconnection varies from satellite pass to satellite pass and within the substorm recovery phase. We find that tail reconnection is an important feature of the recovery phase of substorms.

Moses, J. J.↗
NASA NTRS2002

Control of Jupiter's Radio Emission and Aurorae by the Solar Wind

Radio emissions from Jupiter provided the first evidence that this giant planet has a strong magnetic field and a large magnetosphere. Jupiter also has polar aurorae, which are similar in many respects to Earth's aurorae. The radio emissions are believed to be generated along the high-latitude magnetic field lines by the same electrons that produce the aurorae, and both the radio emission in the hectometric frequency range and the aurorae vary considerably. The origin of the variability, however, has been poorly understood. Here we report simultaneous observations using the Cassini and Galileo spacecraft of hectometric radio emissions and extreme ultraviolet auroral emissions from Jupiter. Our results show that both of these emissions are triggered by interplanetary shocks propagating outward from the Sun. When such a shock arrives at Jupiter, it seems to cause a major compression and reconfiguration of the magnetosphere, which produces strong electric fields and therefore electron acceleration along the auroral field lines, similar to the processes that occur during geomagnetic storms at the Earth.

Gurnett, D. A.↗
NASA NTRS2003

Reduction and Analysis of Data from the IMP 8 Spacecraft

The IMP 8 spacecraft was launched in 1973 and the MIT solar wind Faraday Cup experiment continues to produce excellent data except for a slightly increased noise level. Those data have been important for determining the solar wind interaction with Earth's magnetic field; studies of interplanetary shocks; studies of the correlation lengths of solar wind features through comparisons with other spacecraft; and more recently, especially important for determination of the regions in which the Wind spacecraft was taking data as it passed through Earth's magnetotail and for understanding the propagation of solar wind features from near 1 AU to the two Voyager spacecraft.

Source record↗
NASA NTRS2005

Overview of Results from the Cassini Magnetospheric Imaging Instrument (MIMI) During the First Year of Operations

The MIMI investigation comprises three sensors covering the indicated energy ranges: the Ion and Neutral Camera (INCA) -- 7 keV/nuc 3 Mev) and protons (1.6 < E < 160 Mev) from the back end of the dual field-of-view telescope. The Saturn observation sequences began in January, 2004 and culminated in Saturn Orbit Insertion on July 1, 2004. The MIMI sensors observed substantial activity in interplanetary space for several months prior to SOI, including several interplanetary shocks associated with corotating interaction regions, numerous increases most likely originating from particle streams in the vicinity of the Saturnian bow shock and energetic neutral atoms (ENA) emanating from Saturn s magnetosphere. Results following SOI revealed: a dynamical magnetosphere with a day-night asymmetry and an 11-hour periodicity; several water-product ions (O+, OH+, H2O+), but little N+; inferred quantities of neutral gas sufficient to cause major losses in the trapped ions and electrons in the middle and inner magnetosphere; a Titan exosphere that is a copious source of ENA; INCA imaging through ENA has also revealed a previously unknown radiation belt residing inward of the D-ring that is most likely the result of double charge-exchange between the main radiation belt and the upper layers of Saturn s exosphere. Finally, there is ample evidence for the presence of substorm-like injections of plasma that subsequently corotates for a number of days before dissipating on the night-side magnetotail. The observations will be presented and discussed in the context of current theoretical models.

Krimigis, S. M.↗
NASA NTRS2007

A Survey of Interplanetary Coronal Mass Ejections During 1996 - 2007

Interplanetary coronal mass ejections, the interplanetary counterparts of coronal mass ejections at the Sun, are the major drivers of interplanetary shocks in the heliosphere, and are associated with modulations of the galactic cosmic ray intensity, both short term (Forbush decreases caused by the passage of the shock, post-shock sheath, and ICME) and possibly with longer term modulation. Using several in-situ signatures of ICMEs, including plasma temperature, and composition, magnetic fields, and cosmic ray modulations, made by near-Earth spacecraft, we have compiled a "comprehensive" list of ICMEs passing the Earth since 1996, encompassing solar cycle 23. We summarize the properties of these ICMEs, such as their occurrence rate, speeds, association with solar energetic particle events, shocks and cosmic ray decreases.

Richardson, Ian↗
NASA NTRS2008

The Solar Energetic Particle Event of December 14, 2006

We report on observations of a solar energetic particle event by instruments on five different spacecraft: the Advanced Composition Explorer (ACE), STEREO A and B, WIND, and GOES II. The event began with a class X1.5 .soft x-ray flare in AR930 on December 14 at 22:15 UT. At this time the two STEREO spacecraft were located outside the Earth's magnetosphere and were heading for their first lunar swing-by on December 15. The x-ray event was located on the sun at W46, a longitude which is nominally well-connected magnetically to the Earth. An interplanetary shock, associated with an earlier X3.4 event on December 13 (also from AR930), passed the Earth on December 14 at approx.13:56 (time at ACE). The corresponding magnetic cloud arrived at approx.22 UT on December 14, close to the time of the onset of the particle event associated with the X1.5 flare, and extended until approx.08 UT on December 15. The intensity of approx.14 MeV protons at STEREO A shows three dips by factors of approx.10 or more during the early stages of this event while the spacecraft was within the magnetic cloud. Similar dips are seen for protons to at least 100 MeV. In principle, these dips could have been caused by changes in the magnetic field direction with respect to the particle telescope's field of view while viewing a beamed particle distribution. However, this possibility can be ruled out because the magnetic field at the spacecraft shows no evidence of such directional variations, and similar particle intensity dips were seen by instruments on other spacecraft in the near-Earth solar wind. We shall present evidence that the dips were actually associated with varying magnetic connection to the Sun within the magnetic cloud. These dips were not observed at GOES II, suggesting they were somehow smoothed out by passage into the magnetosphere.

vonRosenvinge, T. T.↗
NASA NTRS2008

Ion Anisotropy and High-Energy Variability of Large Solar Particle Events: A Comparative Study

We have made comparative studies of ion anisotropy and high-energy variability of solar energetic particle (SEP) events previously examined by the Solar, Heliospheric, and Interplanetary Environment (SHINE) Workshop campaign. We have found distinctly different characteristics of SEPs between two large "gradual" events having very similar solar progenitors (the 2002 April 21 and August 24 events). Since the scattering centers of SEPs are approximately frozen in the solar wind, we emphasize work in the solar-wind frame where SEPs tend to be isotropized, and small anisotropies are easier to detect. While in the August event no streaming reversal occurred, in the April event the field-aligned anisotropy of all heavy ions showed sign of streaming reversal. The difference in streaming reversal was consistent with the difference in the presence of the outer reflecting boundary. In the April event the magnetic mirror, which was located behind the interplanetary shock driven by the preceding coronal mass ejection (CME), could block the stream of SEPs, while in the August event SEPs escaped freely because of the absence of nearby boundary. The magnetic mirror was formed at the bottleneck of magnetic field lines draped around a flank of the preceding CME. In the previous SHINE event analysis the contrasting event durations and Fe/O ratios of the both events were explained as the interplay between shock geometry and seed population. Our new findings, however, indicate that event duration and time as well as spectral variation are also affected by the presence of a nearby reflecting boundary.

Tan, Lun C.↗
NASA NTRS2011

Coronal Mass Ejections and Their Heliospheric Consequences

Coronal mass ejections (CMEs) have been recognized as the most energetic phenomenon in the heliosphere, deriving their energy from the stressed magnetic fields on the Sun. The Solar and Heliospheric Observatory (SOHO) mission has accumulated a wealth of information on CMEs with unprecedented extent, clarity, and uniformity. The CME data base has been useful in confirming results from past missions and obtaining a number of new results on the initiation, propagation, and heliospheric consequences of CMEs. This paper highlights some of the recent results on the morphological, physical, and kinematic properties of CMEs observed during solar cycle 23. White-light signatures of CMEs and their utility in inferring coronal magnetic fields near the Sun will be discussed. Presence of coronal holes near eruption regions have been shown to significantly modify the trajectories of CMEs. New results on the interaction of CMEs with large-scale structures such as coronal holes and other CMEs will be presented. Finally, the CME consequences in the heliosphere such as interplanetary shocks, type II radio bursts, energetic particles, geomagnetic storms, and cosmic ray modulation will be discussed

Gopalswamy, Nat↗
NASA NTRS2011

Voyager Observations of Magnetic Fields and Cosmic Rays in the Heliosheath

The major features of the profile of >70 MeV/nuc cosmic ray intensity (CRI) observed by Voyager 1 (V1) in the heliosheath from 2005.8 - 2010.24 are described by the empirical "CR-B" relation as the cumulative effect of variations of the magnetic field strength B. The CRI profile observed by Voyager 2 (V2) from 2008.60 to 2010.28 in the heliosheath is also described by the CR-B relation. On a smaller scale, of the order of a hundred days, a sequence of 3 CRI decreases observed by V1 during 2006 was interpreted as the effect of a propagating interplanetary shock first interacting with the termination shock, then moving past V1, and finally reflecting from the heliopause and propagating back to V1. Our observations show that the second CRI decrease in this sequence began during the passage of a "Global Merged Interaction Region" (GMIR), approx. 40 days after the arrival of the GMIR and its possible shock. The first and third CRI decreases in the sequence were associated with local enhancements of B. The magnetic field observations associated with the second sequence of 3 cosmic ray intensity decreases observed by V1 in 2007/2008 are more difficult to reconcile with the scenario of Webber et al. and the CR-B relation. The discrepancy might indicate the importance of latitudinal effects.

Burlaga, L. F.↗
NASA NTRS2011

Voyager Observations of Magnetic Fields and Cosmic Rays in the Heliosheath

The major features of the profile of greater than 70 MeV/nuc cosmic ray intensity (CRI) observed by Voyager 1 (VI) in the heliosheath from 2005.8-2010.24 are described by the empirical "CR-B" relation as the cumulative effect of variations of the magnetic field strength B. The CRI profile observed by Voyager 2 (V2) from 2008.60 to 2010.28 in the heliosheath is also described by the CR-B relation. On a smaller scale, of the order of a hundred days, a sequence on CRI decreases observed by V 1 during 2006 was interpreted as the effect of a propagating interplanetary shock first interacting with the termination shock, then moving past V1, and finally reflecting from the heliopause and propagating back to V1. Our observations show that the second CRI decrease in this sequence began during the passage of a "Global Merged Interaction Region" (GMIR), ~40 days after the arrival of the GMIR and its possible shock. The first and third CRI decreases in the sequence were associated with local enhancements of B. The magnetic field observations associated with the second sequence of 3 cosmic ray intensity decreases observed by V 1 in 2007/2008 are more difficult to reconcile with the scenario of Webber et al. (2009) and the CR-B relation. The discrepancy might indicate the importance of latitudinal effects

Burlaga, L. F.↗