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NASA NTRS2004

Solar Particle Events at Mars Seen by MARIE, 2002-2003

The MARIE detector aboard the 200 1 Mars Odyssey operated continuously between March 13, 2002, and October 28, 2003. During these twenty months, approximately 12 periods of enhanced solar particle activity were observed. The last of these, on October 26-28, 2003, caused the instrument to lose communication with the spacecraft. In addition to that very large event, several other major Solar Particle Events (SPEs) were observed. Those prior to March 2003 have been discussed elsewhere [I] and will be reviewed here. Many of the solar events seen at Mars correspond well to observations by GOES and ACE satellites near Earth. Some SPEs were observed by MARIE when Mars was on, or near, the opposite side of the sun as seen from Earth; and while a few of them - most notably the large full-halo Coronal Mass Ejections of July 2002 - coincided with events seen by the near-Earth satellites, others were not seen at Earth. Conversely, in this same time period, some SPEs were seen at Earth but not at Mars. Subsequent to February 2003, three SPEs were observed by MARIE, on March 18- 20, May 28 - June 3, and Oct. 26-28. Observations of solar energetic particles at Mars can be combined with data from other points in the heliosphere from, e.g., the Solar Heliospheric Observatory, ACE/CRIS, and GOES satellites near Earth, and Ulysses. Having several different observation points is useful for testing models of CME initiation and propagation.

Cleghorn, T.↗
NASA NTRS2010

Coronal Mass Ejections: a Summary of Recent Results

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. This paper highlights some of the recent results on CMEs obtained from the Solar and Heliospheric Observatory (SOHO) and the Solar Terrestrial Relations Observatory (STEREO) missions. The summary of the talk follows. SOHO observations revealed that the CME rate is almost a factor of two larger than previously thought and varied with the solar activity cycle in a complex way (e.g., high-latitude CMEs occurred in great abundance during the solar maximum years). CMEs were found to interact with other CMEs as well as with other large-scale structures (coronal holes), resulting in deflections and additional particle acceleration. STEREO observations have confirmed the three-dimensional nature of CMEs and the shocks surrounding them. The EUV signatures (flare arcades, corona) dimming, filament eruption, and EUV waves) associated with CMEs have become vital in the identification of solar sources from which CMEs erupt. CMEs with speeds exceeding the characteristic speeds of the corona and the interplanetary medium drive shocks, which produce type II radio bursts. The wavelength range of type II bursts depends on the CME kinetic energy: type II bursts with emission components at all wavelengths (metric to kilometric) are due to CMEs of the highest kinetic energy. Some CMEs, as fast as 1600 km/s do not produce type II bursts, while slow CMEs (400 km/s) occasionally produce type II bursts. These observations can be explained as the variation in the ambient flow speed (solar wind) and the Alfven speed. Not all CME-driven shocks produce type II bursts because either they are subcritical or do not have the appropriate geometry. The same shocks that produce type II bursts also produce solar energetic particles (SEPs), whose release near the Sun seems to be delayed with respect to the onset of type II bursts. This may indicate a subtle difference in the acceleration of the ions and 10 keV electrons needed to produce type II bursts. Surprisingly, some shocks lacking type II bursts are associated with energetic storm particle events (ESPs) pointing to the importance of electron escape from the shock for producing the radio emission. CMEs slow down or accelerate in the interplanetary medium because of the drag force, which modifies the transit time of CMEs and shocks. Halo CMEs that appear to surround the occulting disk were known before the SOHO era as occasional events. During the SOHO era, they became very prominent because of their ability to impact Earth and producing geomagnetic storms. Halo CMEs are generally more energetic than ordinary CMEs, which means they can produce severe impact on Earth's magnetosphere. Their origin close to the disk center of the Sun ensures direct impact on the magnetosphere, although their internal magnetic structure is crucial in causing storms. The solar sources of CMEs that produce SEP events at Earth, on the other hand, are generally in the western hemisphere because of the magnetic connectivity. Thus, CMEs are very interesting from the point of view of plasma physics as well as practical implications because of their space weather impact.

Gopalswamy, Nat↗
NASA NTRS2017

Flux Cancelation: The Key to Solar Eruptions

Solar coronal jets are magnetically channeled eruptions that occur in all types of solar environments (e.g. active regions, quiet-Sun regions and coronal holes). Recent studies show that coronal jets are driven by the eruption of small-scare filaments (minifilaments). Once the eruption is underway magnetic reconnection evidently makes the jet spire and the bright emission in the jet base. However, the triggering mechanism of these eruptions and the formation mechanism of the pre-jet minifilaments are still open questions. In this talk, mainly using SDO/AIA (Solar Dynamics Observatory / Atmospheric Imaging Assembly) and SDO/HIM (Solar Dynamics Observatory / Helioseismic and Magnetic Imager) data, first I will address the question: what triggers the jet-driving minifilament eruptions in different solar environments (coronal holes, quiet regions, active regions)? Then I will talk about the magnetic field evolution that produces the pre-jet minifilaments. By examining pre-jet evolutionary changes in line-of-sight HMI magnetograms while examining concurrent EUV (Extreme Ultra-Violet) images of coronal and transition-region emission, we find clear evidence that flux cancelation is the main process that builds pre-jet minifilaments, and is also the main process that triggers the eruptions. I will also present results from our ongoing work indicating that jet-driving minifilament eruptions are analogous to larger-scare filament eruptions that make flares and CMEs (Coronal Mass Ejections). We find that persistent flux cancellation at the neutral line of large-scale filaments often triggers their eruptions. From our observations we infer that flux cancelation is the fundamental process from the buildup and triggering of solar eruptions of all sizes.

Panesar, Navdeep K.↗
NASA NTRS1992

Coronal shock waves

The formation of magnetohydrodynamic (MHD) shocks near the leading edge of Coronal Mass Ejections (CME's) is considered using results from both shock theory and numerical simulations. By using just the shock theory results and a simplified model corona, the following is shown: a slow shock front should be concave upward (away from the solar surface); a configuration containing intermediate shocks should also be concave upward near the center of the CME and concave downward at some distance from the CME centerline; and a fast shock configuration should be concave downward. These results are verified with numerical simulations of the MHD equations for the propagation of shocks through a quiescent coronal streamer. By examining the characteristic wave speeds in the ambient streamer, it is estimated that the various shock configurations should form for CME speeds within the following speed increments: slow shocks, 200 to 300 km/sec; intermediate shocks, 300 to 900 km/sec; and fast shocks, greater than 900 km/sec.

Steinolfson, Richard S.↗
NASA NTRS1987

ISEE 3 observations of low-energy proton bidirectional events and their relation to isolated interplanetary magnetic structures

The paper represents the results of a comprehensive survey of low-energy proton bidirectional anisotropies and associated transient magnetic structures as observed in the 35-1600 keV energy range on ISEE-3 during the last solar maximum. The majority of observed bidirectional flow (BDF) events (more than 70 percent) are associated with isolated magnetic structures which are postulated to be an interplanetary manifestation of coronal mass ejection (CME) events. The observed BDF events can be qualitatively grouped into five classes depending on the field signature of the related magnetic structure and the association (or lack of association) with an interplanetary shock. Concerning the topology of the CME-related magnetic structures, the observations are interpreted as being consistent with a detached bubble, comprising closed loops or tightly wound helices.

Marsden, R. G.↗
NASA NTRS2005

Coronal Current Sheet Evolution in the Aftermath of a CME

We report on SOHO-UVCS observations of coronal restructuring following a Coronal Mass Ejection (CME) on November 26, 2002, at the time of a SOHO-Ulysses quadrature campaign. Starting about 3 hours after the CME, which was directed towards Ulysses, UVCS began taking spectra at 1.7 solar radii, covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 Angstrom line of [Fe XVIII], indicating temperatures above 6x10(6) K, was observed throughout the campaign in a spatially limited location. Comparison with EIT images shows the [Fe XVIII] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fe XVIII] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of our observations: in particular, we give the temperature vs. time in the current sheet and estimate the density. Ulysses was directly above the location of the CME and intercepted the ejecta. High ionization state Fe was detected by SWICS throughout the magnetic cloud associated with the CME, although the rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. Both the remote and in situ observations are compared with predictions of theoretical CME models.

Bemporad, A.↗
NASA NTRS1994

On the sources of interplanetary shocks at 0.72 AU

In order to understand the solar cycle variation of interplanetary shocks and their driving source at 0.72 AU, a survey of Pioneer Venus Orbiter (PVO) magnetometer and plasma data from 1979-1988 has been conducted. Known shock drivers at 1.0 AU include coronal mass ejections (CMEs) and fast/slow stream interactions. In our analysis, CMEs were identified by a decrease in plasma temperature to background or below accompanied by an increase in plasma density and dynamic pressure. It was also required that the magnetic field exhibit a coherent rotation over about a day and an increase and decline in magnitude on a timescale of hours to days. Stream interactions were identified by a characteristic increase in ion temperature and velocity coincident with a decrease in density and a coincident increase in the total magnetic field magnitude. These signatures were usually preceded within 24 hours by a change in flow angle. In all, 45 shocks were identified: 36 driven by CMEs, 6 resulting from fast/slow stream interactions, and 3 with sources that could not be defined. The shocks driven by CMEs show a solar cycle variation that roughly follows the sunspot number. These shocks all have normals consistent with radial propagation of the shock fronts from the sun. In contrast, the few stream interaction related shocks show a tendency to occur later in the solar cycle and have a broader distribution of shock normals.

Lindsay, G. M.↗
NASA NTRS2010

Constraints on CME Evolution from in situ Observations of Ionic Charge States

We present a novel procedure for deriving the physical properties of Coronal Mass Ejections (CMES) in the corona. Our methodology uses in-situ measurements of ionic charge states of C, O, Si and Fe in the heliosphere and interprets them in the context of a model for the early evolution of ICME plasma, between 2 - 5 R-solar. We find that the data can be fit only by an evolution that consists of an initial heating of the plasma, followed by an expansion that ultimately results in cooling. The heating profile is consistent with a compression of coronal plasma due to flare reconnect ion jets and an expansion cooling due to the ejection, as expected from the standard CME/flare model. The observed frozen-in ionic charge states reflect this time-history and, therefore, provide important constraints for the heating and expansion time-scales, as well as the maximum temperature the CME plasma is heated to during its eruption. Furthermore, our analysis places severe limits on the possible density of CME plasma in the corona. We discuss the implications of our results for CME models and for future analysis of ICME plasma composition.

Gruesbeck, Jacob R.↗
NASA NTRS1991

Flares on the sun and other stars

Research on solar and stellar flares is reviewed. The characteristics of flares in various spectral regions are described, and the physics of flares is discussed, including the two-ribbon flare model, coronal mass ejections, flare dynamics, white light flare production, and electron beams and proton beams in hard X-ray flares. The analysis of the flare energy budget, magnetic field measurements, multispectral stellar observations, plasma emissivity, conditions in flare loops, cooling curves, and proton beam signatures is addressed. Microflaring and coronal heating are examined.

Haisch, Bernhard↗
NASA NTRS1993

Solar cycle 21 effects on the Interplanetary Magnetic Field and related parameters at 0.7 and 1.0 AU

Magnetometer data obtained over the course of the previous solar cycle by the Pioneer Venus orbiter at about 0.7 AU and IMP 8 at 1.0 AU are used to compare the long-term behavior of the IMF at these two heliocentric distances. Similarities include an enhancement in the typical or median field magnitude during the declining phase of the solar cycle (as compared to solar maximum or minimum), slight decreases in the Parker spiral angle from the declining phase through solar minimum, similar trends in the Alfvenic and magnetosonic Mach numbers, and a remarkably consistent sector structure. Differences include the temporal behavior of the high-field tail of the field distribution, showing that high fields are most frequently observed during solar maximum at the Earth but during the declining phase of activity at Venus. This feature suggests that the perceived occurrence history of large fields from transient disturbances such as coronal mass ejections is a sensitive function of position within the heliosphere.

Luhmann, J. G.↗
NASA NTRS1993

The solar flare myth

Many years of research have demonstrated that large, nonrecurrent geomagnetic storms, shock wave disturbances in the solar wind, and energetic particle events in interplanetary space often occur in close association with large solar flares. This result has led to a pradigm of cause and effect - that large solar flares are the fundamental cause of these events in the near-Earth space environmemt. This paradigm, which I call 'the solar flare myth,' dominates the popular perception of the relationship between solar activity and interplanetary and geomagnetic events and has provided much of the pragmatic rationale for the study of the solar flare phenomenon. Yet there is good evidence that this paradigm is wrong and that flares do not generally play a central role in producing major transient disturbances in the near-Earth space environment. In this paper I outline a different paradigm of cause and effect that removes solar flares from their central position in the chain of events leading from the Sun to near-Earth space. Instead, this central role is given to events known as coronal mass ejections.

Gosling, J. T.↗
NASA NTRS2008

Coronal Mass Ejections and their Implications for the Corona and Heliosphere

Coronal mass ejections (CMEs) are the largest and most energetic form of transients that connect the Sun to the heliosphere. They are critically important both for understanding the physical mechanisms of explosive solar activity and for predicting space weather. Furthermore they are an extreme example of how cross-scale coupling can play a critical role in determining the properties of a large-scale dynamical system. In this presentation CME theories are reviewed and the latest results from 3D numerical modeling of CME initiation propagation to the heliosphere are presented. In particular the focus is on the breakout model, but many of the results hold for the flux rope models as well. The implications of these results for understanding heliospheric structure and dynamics and for upcoming space missions will be discussed.

Antiochos, Spiro K.↗
NASA NTRS2008

Coronal Mass Ejections - A Statistical View

Although first recognized in 1971, the quasi-continuous record since 1979 of the appearance of coronal mass ejections (CMEs-perhaps more appropriately called coronal magnetic ejections) has resulted in a stable understanding of their properties, at least from a statistical viewpoint. These eruptions occur every few days during solar activity minimum and many times per day during maximum. They are believed to play an important role throughout the heliosphere in such diverse events as removing helicity from the corona; modulating the energetic particle environment in the inner heliosphere; causing severe geomagnetic storms at Earth and other magnetic bodies throughout the solar system; and controlling the galactic cosmic ray flux. It is therefore understandable that researchers have studied both individual events and the ensemble of CMEs observed over several solar cycles. We will present an overview of these statistics, some new recent observations, and a personal perspective on potential paths of future research.

SaintCyr, O. C.↗
NASA NTRS2017

Using the Coronal Evolution to Successfully Forward Model CMEs' In Situ Magnetic Profiles

Predicting the effects of a coronal mass ejection (CME) impact requires knowing if impact will occur, which part of the CME impacts, and its magnetic properties. We explore the relation between CME deflections and rotations, which change the position and orientation of a CME, and the resulting magnetic profiles at 1 AU. For 45 STEREO-era, Earth-impacting CMEs, we determine the solar source of each CME, reconstruct its coronal position and orientation, and perform a ForeCAT (Forecasting a CME's Altered Trajectory) simulation of the coronal deflection and rotation. From the reconstructed and modeled CME deflections and rotations, we determine the solar cycle variation and correlations with CME properties. We assume no evolution between the outer corona and 1 AU and use the ForeCAT results to drive the ForeCAT In situ Data Observer (FIDO) in situ magnetic field model, allowing for comparisons with ACE and Wind observations. We do not attempt to reproduce the arrival time. On average FIDO reproduces the in situ magnetic field for each vector component with an error equivalent to 35 percent of the average total magnetic field strength when the total modeled magnetic field is scaled to match the average observed value. Random walk best fits distinguish between ForeCAT's ability to determine FIDO's input parameters and the limitations of the simple flux rope model. These best fits reduce the average error to 30 percent.The FIDO results are sensitive to changes of order a degree in the CME latitude, longitude, and tilt, suggesting that accurate space weather predictions require accurate measurements of a CME's position and orientation.

Kay, C.↗
NASA NTRS2010

Interplanetary Shocks Lacking Type 2 Radio Bursts

We report on the radio-emission characteristics of 222 interplanetary (IP) shocks detected by spacecraft at Sun-Earth L1 during solar cycle 23 (1996 to 2006, inclusive). A surprisingly large fraction of the IP shocks (approximately 34%) was radio quiet (RQ; i.e., the shocks lacked type II radio bursts). We examined the properties of coronal mass ejections (CMEs) and soft X-ray flares associated with such RQ shocks and compared them with those of the radio-loud (RL) shocks. The CMEs associated with the RQ shocks were generally slow (average speed approximately 535 km/s) and only approximately 40% of the CMEs were halos. The corresponding numbers for CMEs associated with RL shocks were 1237 km/s and 72%, respectively. Thus, the CME kinetic energy seems to be the deciding factor in the radio-emission properties of shocks. The lower kinetic energy of CMEs associated with RQ shocks is also suggested by the lower peak soft X-ray flux of the associated flares (C3.4 versus M4.7 for RL shocks). CMEs associated with RQ CMEs were generally accelerating within the coronagraph field of view (average acceleration approximately +6.8 m/s (exp 2)), while those associated with RL shocks were decelerating (average acceleration approximately 3.5 m/s (exp 2)). This suggests that many of the RQ shocks formed at large distances from the Sun, typically beyond 10 Rs, consistent with the absence of metric and decameter-hectometric (DH) type II radio bursts. A small fraction of RL shocks had type II radio emission solely in the kilometric (km) wavelength domain. Interestingly, the kinematics of the CMEs associated with the km type II bursts is similar to those of RQ shocks, except that the former are slightly more energetic. Comparison of the shock Mach numbers at 1 AU shows that the RQ shocks are mostly subcritical, suggesting that they were not efficient in accelerating electrons. The Mach number values also indicate that most of these are quasi-perpendicular shocks. The radio-quietness is predominant in the rise phase and decreases through the maximum and declining phases of solar cycle 23. About 18% of the IP shocks do not have discernible ejecta behind them. These shocks are due to CMEs moving at large angles from the Sun-Earth line and hence are not blast waves. The solar sources of the shock-driving CMEs follow the sunspot butterfly diagram, consistent with the higher-energy requirement for driving shocks.

Gopalswamy, N.↗
NASA NTRS1988

Forward-reverse shock pairs associated with transient disturbances in the solar wind at 1 AU

Using color-coded plots of the ISEE-3 solar wind electron data and magnetic field data from ISEE-3 for the period from August 1978 through February 1980, evidence was obtained on two transient disturbances which contained reverse shocks in addition to forward shocks. These disturbances are considered to be associated with coronal mass ejections (CMEs). In the stronger of the two disturbances, the reverse shock was found within the CME and was separated from the forward shock by about 0.2 AU; the pressure between the two shocks was nearly constant. In the weaker disturbance, the reverse shock propagated entirely through the CME, trailing the forward shock by about 0.3-0.4 AU; the pressure between the shocks declined substantially and monotonically. Each disturbance profile can be compared favorably with one of the simple one-dimensional fluid simulations used by Hundhausen (1985) to illustrate the general principles underlying transient disturbance propagation in the solar wind.

Gosling, J. T.↗
NASA NTRS1986

Solar gradual hard X-ray bursts - Observations and an interpretation

A recent study of solar gradual hard X-ray bursts is summarized. The data are interpreted in terms of a model involving the acceleration and trapping of electrons in post flare loop systems following coronal mass ejections. A controversy about the classification of the metric continuum that typically accompanies gradual hard X-ray events is addressed.

Cliver, E. W.↗