Kristin Simunac

THE VERY UNUSUAL INTERPLANETARY CORONAL MASS EJECTION OF 2012 JULY 23: A BLAST WAVE MEDIATED BY SOLAR ENERGETIC PARTICLES

The giant, superfast, interplanetary coronal mass ejection, detected by STEREO A on 2012 July 23, well away from Earth, appears to have reached 1 AU with an unusual set of leading bow waves resembling in some ways a subsonic interaction, possibly due to the high pressures present in the very energetic particles produced in this event. Eventually, a front of record high-speed flow reached STEREO. The unusual behavior of this event is illustrated using the magnetic field, plasma, and energetic ion observations obtained by STEREO. Had the Earth been at the location of STEREO, the large southward-oriented magnetic field component in the event, combined with its high speed, would have produced a record storm.

Simulation of the 23 July 2012 Extreme Space Weather Event: What if This Extremely Rare CME Was Earth Directed?

Extreme space weather events are known to cause adverse impacts on critical modern day technological infrastructure such as high-voltage electric power transmission grids. On 23 July 2012, NASAs Solar Terrestrial Relations Observatory-Ahead (STEREO-A) spacecraft observed in situ an extremely fast coronal mass ejection (CME) that traveled 0.96 astronomical units (∼1 AU) in about 19 h. Here we use the Space Weather Modeling Framework (SWMF) to perform a simulation of this rare CME. We consider STEREO-A in situ observations to represent the upstream L1 solar wind boundary conditions. The goal of this study is to examine what would have happened if this Rare-type CME was Earth-bound. Global SWMF-generated ground geomagnetic field perturbations are used to compute the simulated induced geoelectric field at specific ground-based active INTERMAGNET magnetometer sites. Simulation results show that while modeled global SYM-H index, a high-resolution equivalent of the Dst index, was comparable to previously observed severe geomagnetic storms such as the Halloween 2003 storm, the 23 July CME would have produced some of the largest geomagnetically induced electric fields, making it very geoeffective. These results have important practical applications for risk management of electrical power grids.

Intermittent release of transients in the slow solar wind: 2. In situ evidence

In paper 1, we showed that the Heliospheric Imager (HI) instruments on the pair of NASA STEREO spacecraft can be used to image the streamer belt and, in particular, the variability of the slow solar wind which originates near helmet streamers. The observation of intense intermittent transient outflow by HI implies that the corresponding in situ observations of the slow solar wind and corotating interaction regions (CIRs) should contain many signatures of transients. In the present paper, we compare the HI observations with in situ measurements from the STEREO and ACE spacecraft. Analysis of the solar wind ion, magnetic field, and suprathermal electron flux measurements from the STEREO spacecraft reveals the presence of both closed and partially disconnected interplanetary magnetic field lines permeating the slow solar wind. We predict that one of the transients embedded within the second CIR (CIR-D in paper 1) should impact the near-Earth ACE spacecraft. ACE measurements confirm the presence of a transient at the time of CIR passage; the transient signature includes helical magnetic fields and bidirectional suprathermal electrons. On the same day, a strahl electron dropout is observed at STEREO-B, correlated with the passage of a high-plasma beta structure. Unlike ACE, STEREO-B observes the transient a few hours ahead of the CIR. STEREO-A, STEREO-B, and ACE spacecraft observe very different slow solar wind properties ahead of and during the CIR analyzed in this paper, which we associate with the intermittent release of transients.

Escape of O+ through the distant tail plasma sheet

[1] In February 2007, the STEREO-B spacecraft encountered the magnetosheath, plasma sheet and plasma sheet boundary layer from about 200 R E to 300 R E downtail. This time period was during solar minimum, and there was no storm activity during this month. Using data from the PLASTIC instrument, we find that even during quiet times, O + is a constant feature of the deep magnetotail, with an O + density of about 15% of the O + density in the near-earth plasma sheet for similar conditions. The tailward flux of the O + is similar to the flux of O + beams that have been observed in the lobe/mantle region of the deep tail. The total outflow rate of the O + down the plasma sheet is 1.1 × 10 24 ions/s, which is 10% of the total outflow rate of 1 × 10 25 ions/s, and of the same order as the estimated loss from dayside transport.

PLASMOID RELEASES IN THE HELIOSPHERIC CURRENT SHEET AND ASSOCIATED CORONAL HOLE BOUNDARY LAYER EVOLUTION

As the heliospheric current sheet (HCS) is corotating past STEREO-B, near-Earth spacecraft ACE, Wind and Cluster, and STEREO-A over more than three days between 2008 January 10 and 14, we observe various sections of (near-pressure-balanced) flux-rope- and magnetic-island-type plasmoids in the associated heliospheric plasma sheet (HPS). The plasmoids can qualify as slow interplanetary coronal mass ejections and are relatively low proton beta (<0.5) structures, with small length scales (an order of magnitude lower than typical magnetic cloud values) and low magnetic field strengths (2-8 nT). One of them, in particular, detected at STEREO-B, corresponds to the first reported evidence of a detached plasmoid in the HPS. The in situ signatures near Earth are associated with a long-decay X-ray flare and a slow small-scale streamer ejecta, observed remotely with white-light coronagraphs aboard STEREO-B and SOHO and tracked by triangulation. Before the arrival of the HPS, a coronal hole boundary layer (CHBL) is detected in situ. The multi-spacecraft observations indicate a CHBL stream corotating with the HCS but with a decreasing speed distribution suggestive of a localized or transient nature. While we may reasonably assume that an interaction between ejecta and CHBL provides the source of momentum for the slow ejectas acceleration, the outstanding composition properties of the CHBL near Earth provide here circumstantial evidence that this interaction or possibly an earlier one, taking place during streamer swelling when the ejecta rises slowly, results in additional mixing processes.

A statistical analysis of properties of small transients in the solar wind 2007–2009: STEREO and Wind observations

We present a comprehensive statistical analysis of small solar wind transients (STs) in 2007–2009. Extending work on STs by Kilpua et al. (2009) to a 3 year period, we arrive at the following identification criteria: (i) a duration < 12 h, (ii) a low proton temperature and/or a low proton beta, and (iii) enhanced field strength relative to the 3 year average. In addition, it must have at least one of the following: (a) decreased magnetic field variability, (b) large, coherent rotation of the field vector, (c) low Alfven Mach number, and (d) Te/Tp higher than the 3 year average. These criteria include magnetic flux ropes. We searched for STs using Wind and STEREO data. We exclude Alfvenic fluctuations. Case studies illustrate features of these configurations. In total, we find 126 examples, ∼81% of which lie in the slow solar wind (≤ 450 km s−1). Many start or end with sharp field and flow gradients/discontinuities. Year 2009 had the largest number of STs. The average ST duration is ∼4.3 h, 75%<6 h. Comparing with interplanetary coronal mass ejections (ICMEs) in the same solar minimum, we find the major difference to be that Tp in STs is not significantly less than the expected Tp. Thus, whereas a low Tp is generally considered a very reliable signature of ICMEs, it is not a robust signature of STs. Finally, since plasma β∼1, force-free modeling of STs having a magnetic flux rope geometry may be inappropriate.

A statistical analysis of heliospheric plasma sheets, heliospheric current sheets, and sector boundaries observed in situ by STEREO

The heliocentric orbits of STEREO A and B with a separation in longitude increasing by about 45 degrees per year provide the unique opportunity to study the evolution of the heliospheric plasma sheet (HPS) on a time scale of up to 2days and to investigate the relative locations of HPSs and heliospheric current sheets (HCSs). Previous work usually determined the HCS locations based only on the interplanetary magnetic field. A recent study showed that a HCS can be taken as a global structure only when it matches with a sector boundary (SB). Using magnetic field and suprathermal electron data, it was also shown that the relative location of HCS and SB can be classified into five different types of configurations. However, only for two out of these five configurations, the HCS and SB are located at the same position and only these will therefore be used for our study of the HCS/HPS relative location. We find that out of 37 SBs in our data set, there are 10 suitable HPS/HCS event pairs. We find that an HPS can either straddle or border the related HCS. Comparing the corresponding HPS observations between STEREO A and B, we find that the relative HCS/HPS locations are mostly similar. In addition, the time difference of the HPSs observations between STEREO A and B match well with the predicted time delay for the solar wind coming out of a similar region of the Sun. We therefore conclude that HPSs are stationary structures originating at the Sun.

He pickup ions in the inner heliosphere-diagnostics of the local interstellar gas and of interplanetary conditions

The relative motion of the Sun through the Local Interstellar Cloud (LIC) leads to a neutral wind through the heliosphere. Because of its high ionization potential, He remains neutral to well within 1 AU, where it is deflected by the Sun’s gravity and forms a focusing cone on the downwind side. This flow pattern has been studied with UV backscattering, through pickup ions (PUI), and atom imaging. A consolidated set of the physical parameters of He in the LIC has been derived combining all three methods. However, it is still poorly understood why PUI fluxes and velocity distributions vary substantially on temporal scales from hours to many days, which leads among other phenomena to apparent changes in the appearance of the focusing cone, even after averaging over several days. With the combination of PLASTIC on STEREO A and B as well as SWICS on ACE, simultaneous PUI observations over an increasing range of heliospheric longitudes have become possible, for which we have initiated a cross‐calibration effort...

Identifying The Ends Of High‐Speed Streams Near 1 AU With In Situ Data From STEREO/PLASTIC

The transition from fast to slow solar wind has not been studied to the same extent as the transition from slow to fast streams: the stream interface. The fast‐to‐slow transition has been reported as a smaller‐scale mirror image of a stream interface. It is characterized by changes in speed, density, temperature, and composition (Geiss et al. 1995; Zurbuchen et al. 1999; Burlaga, Mish, and Whang 1990; Siscoe and Intriligator 1993; Burton et al. 1999). In this study we use solar wind ion data from STEREO/PLASTIC to further investigate the fast‐to‐slow wind interface, with particular emphasis on possible correlations between changes in the proton specific entropy argument and the solar wind’s flow direction. The data were obtained during solar minimum near 1 AU and in the ecliptic plane. We find the fast‐to‐slow transition derived from the change in flow angle is neither as extended as the change in compositional signatures near 1 AU, nor as abrupt as that based on entropy at 4.5 to 5 AU.

Proton enhancement and decreased O6+/H at the heliospheric current sheet: implications for the origin of slow solar wind

We investigated the proton enhancement and O6+/H depletion in the vicinity of the heliospheric current sheet (HCS) using data from STEREO/PLASTIC and STEREO/IMPACT. Three HCS crossing events were studied. For the first two events, the proton enhancement and O6+/H depletion are found to lie at one edge of the HCS. The proton density has a steep slope both at the HCS and at the other boundary of the enhancement. In the third event the proton enhancement and O6+/H depletion surround the HCS and last for 8 hours while the density profile is very different from the other two events. Velocity shear is observed at the HCS for the first two events but not for the third. The enhancement of hydrogen and depletion of oxygen at the streamer belt in the solar corona have been reported using UVCS observation. A potential connection with our observations is based on the similar features observed at 1 AU. How the plasma flows out of the streamer belt, and why there are different features in HCS encounters remain open que...