K. D. C. Simunac

University of New Hampshire, Durham, New Hampshire, United States

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Publications (76)93.36 Total impact

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    ABSTRACT: The heliocentric orbits of STEREO A and B with a separation in longitude increasing by about 45° per year provide the unique opportunity to study the evolution of the heliospheric plasma sheet (HPS) on a time scale of up to ~2 days 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 HPS can be classified into five different types of configurations. However, only for two out of these five configurations the HCS and HPS are located at the same position and only these will therefore be used for our study of the HCS/HPS relative location. We find out of 37 SBs in our dataset 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.
    Journal of Geophysical Research: Space Physics 11/2014; 119(11). DOI:10.1002/2014JA019956 · 3.44 Impact Factor
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    ABSTRACT: 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 Alfvén 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 Alfvénic 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). 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.
    Journal of Geophysical Research Atmospheres 02/2014; 119(2):689-708. DOI:10.1002/2013JA019115 · 3.44 Impact Factor
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    ABSTRACT: [1] 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, NASA's 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.
    Space Weather 12/2013; DOI:10.1002/2013SW000990 · 2.22 Impact Factor
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    ABSTRACT: key goal for space weather studies is to define severe and extreme conditions that might plausibly afflict human technology. On 23 July 2012, solar active region 1520 (~141°W heliographic longitude) gave rise to a powerful coronal mass ejection (CME) with an initial speed that was determined to be 2500 ± 500 km/s. The eruption was directed away from Earth toward 125°W longitude. STEREO-A sensors detected the CME arrival only about 19 h later and made in situ measurements of the solar wind and interplanetary magnetic field. In this paper, we address the question of what would have happened if this powerful interplanetary event had been Earthward directed. Using a well-proven geomagnetic storm forecast model, we find that the 23-24 July event would certainly have produced a geomagnetic storm that was comparable to the largest events of the twentieth century (Dst ~ -500 nT). Using plausible assumptions about seasonal and time-of-day orientation of the Earth's magnetic dipole, the most extreme modeled value of storm-time disturbance would have been Dst = -1182 nT. This is considerably larger than estimates for the famous Carrington storm of 1859. This finding has far reaching implications because it demonstrates that extreme space weather conditions such as those during March of 1989 or September of 1859 can happen even during a modest solar activity cycle such as the one presently underway. We argue that this extreme event should immediately be employed by the space weather community to model severe space weather effects on technological systems such as the electric power grid.
    Space Weather 10/2013; 11(10):585-591. DOI:10.1002/swe.20097 · 2.22 Impact Factor
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    ABSTRACT: The topic of suprathermal and energetic ion events upstream of the Earth's bow shock has been investigated since the late 1960's. Over the past 50 years, these events have been characterized as having energies ranging from just above the solar wind energies on through 2MeV, time spans of minutes to hours, and particle distributions ranging from field aligned to isotropic. The seed particles of these events accelerated within the magnetosphere and/or at the Earth's bow shock have been shown to be of ions originating in the magnetosphere, solar wind, as well as ions energized in other heliospheric processes (such as solar energetic particle (SEP), corotating interaction regions (CIRs), Pick-up ions, etc.). In this study we utilize STEREO/PLASTIC to examine bow shock/magnetospheric energetic proton events observed throughout 2007 in the region far upstream of the Earth's ion foreshock. To do this, we first employ an automated procedure to identify suprathermal proton events in the energy range of 4keV up to 80keV. The occurrence of events, magnetic connection to the Earth, and Compton-Getting transformed energy spectra of 66 possible STA bow shock/magnetospheric events are investigated as a function of spacecraft-Earth separation.
    06/2013; DOI:10.1063/1.4811065
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    ABSTRACT: 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.
    The Astrophysical Journal 06/2013; 770:38. DOI:10.1088/0004-637X/770/1/38 · 6.28 Impact Factor
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    ABSTRACT: Interstellar neutral helium penetrates to well within 1 AU where it is ionized predominantly by UV from the Sun. These freshly created He+ pickup ions (PUI) are convected outward with the solar wind and subject to acceleration processes, for example in compression regions, corotating interaction regions, or CME-driven interplanetary shocks. These acceleration processes result in the formation of suprathermal tails in the PUI velocity distribution. The PLASTIC instruments on the STEREO spacecraft cover for pickup He the energy range of ~1-20 keV/nuc. They allow the determination of PUI velocity distribution functions from the source distribution to suprathermal energies. In this paper we study the suprathermal tails of He pickup ions during the period January to December 2008 that featured many corotating interaction regions. The PLASTIC observations for the He+ suprathermal tail show a large variability of the spectral index γ of the distribution function f(w)~wγ, with w = V/Vsw, where V and Vsw are the particle velocity and the solar wind velocity, respectively. Incorporating a transformation from the spacecraft frame to the solar wind frame, the spectral index at velocities 1.5 <= w <= 5 (in the solar wind frame) ranges from γ ~ -5.7 at a forward shock to γ = -9 in the slow solar wind.
    06/2013; DOI:10.1063/1.4811036
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    ABSTRACT: This recent solar minimum and the rise of solar cycle 24 in 2007-2011 have presented conditions on the solar surface that have made the existence of multiple streamers and low-to-mid latitude coronal holes a rule rather than the exception for this relatively low solar activity period. The result has been many contributions to the solar wind from a corona that appears more like the solar maximum corona from a magnetic field geometry standpoint, but at the same time generally lacks the increased heating associated with the emergence and evolution of the strong field active regions. We use the PFSS coronal field model to infer the distinctive characteristics of the recent weak solar minimum and early rising activity phase sources compared to previous stronger cycles at comparable phases. We then use the model to illustrate that these sources occur where the coronal holes are continually evolving with the surface flux via differential rotation and other convective and diffusive actions near the solar surface. The results suggest that not only does most of the ecliptic wind come from the lower latitude coronal holes and an irregular polar hole boundary, but that these regions are also routinely reconfiguring to keep up with the evolving surface field conditions-implying transients should make regular contributions to the ecliptic wind in this period . A picture of the location and evolution of the ecliptic mappings to this transient layer from 2007 to 2011 is constructed and potential observational consequences for future investigation are suggested.
    06/2013; DOI:10.1063/1.4811001
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    ABSTRACT: We have observed the solar wind extensively using the twin STEREO spacecraft in 2007 - 2011, covering the deep solar minimum 23/24 and the rising phase of solar cycle 24. Hundreds of large-scale solar wind structures have been surveyed, including stream interaction regions (SIRs), interplanetary CMEs (ICMEs), and interplanetary shocks. The difference in location can cause one STEREO spacecraft to encounter 1/3 more of the above structures than the other spacecraft in a single year, even of the quasi-steady SIRs. In contrast with the rising phase of cycle 23, SIRs and ICMEs have weaker field and pressure compression in this rising phase, and ICMEs drive fewer shocks. Although the majority of shocks are driven by SIRs and ICMEs, we find ~13% of shocks without clear drivers observed in situ.
    06/2013; DOI:10.1063/1.4811020
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    ABSTRACT: Year 2009 was the last year of a long and pronounced solar activity minimum. In this year the solar wind in the inner heliosphere was for 90% of the time slow (< 450 km s-1) and with a weaker magnetic field strength compared to the previous solar minimum 1995-1996. We choose this year to present the results of a systematic search for small solar wind transients (STs) observed by the STEREO-Ahead (ST-A) probe. The data are from the PLASTIC and IMPACT instrument suites. By "small" we mean a duration from ~1 to 12 hours. The parameters we search for to identify STs are (i) the total field strength, (ii) the rotation of the magnetic field vector, (iii) its smoothness, (iv) proton temperature, (v) proton beta, and (vi) Alfvén Mach number. We find 45 examples. The STs have an average duration of ~4 hours. Ensemble averages of key quantities are: (i) maximum B = 7.01 nT; (ii) proton β = 0.18; (iii) proton thermal speed = 20.8 km s-1 and (iv) Alfvén Mach number = 6.13. No distinctive feature is found in the pitch angle distributions of suprathermal electrons. Our statistical results are compared with those of STs observed near Earth by Wind during 2009.
    06/2013; DOI:10.1063/1.4811050
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    ABSTRACT: In August 2011 Comet Elenin passed STEREO B spacecraft to within just 7 million kilometers (or 0.046AU). The comet's trajectory was almost in the same plane as the Earth's orbit and hence of the STEREO B spacecraft. STEREO B was in the tail region of the comet for at least a week. The comet continued pass the spacecraft and went inside the Earth's orbit closer to the Sun, during which the distance of STB and the comet increased. This configuration gave us the unique opportunity to study the composition of the comet tail and the temporal/spatial evolution of the comet's ion distribution for an extended 3-week time period. During the tail passage we observed suprathermal pickup ions H+ and He+ and singly charged water group ions (which are a clear signature of comet material). In this presentation we will report on the temporal/spatial evolution of the suprathermal ions within an energy range of 0.3keV/q to 80keV/q, the composition and their energy spectra during the passage of the comet.
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    ABSTRACT: During the solar activity minimum 2007-2009 very few large transients (interplanetary coronal mass ejections, ICMEs) were observed. However, an interesting feature was the frequent occurrence of small transients (few hours' duration), as shown, for example over a two-month interval by Kilpua et al. (2009). In this work we present a comprehensive statistical analysis of small transients over the entire three-year period. Identification crieria are : (i) duration between 0.5 and 12 hours; (ii) low proton temperature; (iii) low proton beta; (iv) enhanced magnetic field strength; (v) diminished magnetic field variability; (vi) low Alfven Mach number; and (vii) higher-than-average over the three years of the electron-to-proton temperature ratio. In selecting events, we require small transients to satisfy criteria (i)-(iii) and, in addition, they should satisfy at least two of the other four signatures. We compare their properties with those of the solar wind during the same three-year period, and are thus able to isolate a number of features characterizing these small transients during this solar activity minimum period. We search for small transients using observations acquired by the Wind spacecraft. After removing those which are likely to be Alfvenic strctures, we find 131 examples, about 81 percent of which lie in the slow solar wind (< 450 km/s). We present six case studies to illustrate various interesting aspects of these configurations. We then give statistical results on the whole assembly. The average duration is about 4.3 hours, while 99 events (76 pervent) are shorter than 6 hours. The maximum magnetic field is about twice that of the average solar wind value, the proton beta is about four times smaller than ambient, and the Alfven Mach number is about one half of the average value.
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    ABSTRACT: Mirror-mode structures have been found in the solar wind at various heliocentric distances with different missions. Recently, STEREO has observed mirror-mode waves present as trains of holes and also as humps in the magnetic field magnitude. In some cases, mirror-mode trains last for very long periods of time and have been called "mirror-mode storms". We present case studies of mirror-mode storms observed in the solar wind using STEREO data in three different locations: in the downstream region of the forward shock of a stream interaction region, inside a stream interaction region far from the forward shock, and also in the ambient solar wind. To make a formal identification of the mirror mode, we determine wave characteristics using minimum variance analysis. Finally, we perform a kinetic dispersion analysis and discuss the possible origin of mirror-mode structures evaluating curves of growth for different regimes of proton temperature anisotropies in a plasma with a He component.
    Journal of Geophysical Research Atmospheres 01/2013; 118(1):17-28. DOI:10.1029/2012JA018233 · 3.44 Impact Factor
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    ABSTRACT: The solar corona, the outermost portion of the gravitationally bound solar atmosphere, provides the bulk of the interplanetary medium and creates the larger heliosphere- and yet the details of how its properties map outward appear increasingly complex. While coronal holes, the magnetically open portions of the solar magnetic field, have been well-demonstrated to correlate with major solar wind streams observed at 1 AU in the ecliptic, recent modeling and observations indicate the importance of closed magnetic field regions on the Sun as well. The balance between the open and closed field regions is directly determined by the solar dynamo and coronal heating processes and is in a constantly evolving state through the solar activity cycle. We use observations and models to suggest the contributions of closed coronal field regions to the recent and past solar wind. This picture, bolstered by solar images, is particularly important for the low latitude heliosphere where the planets experience the consequences.
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    ABSTRACT: Observations of the solar wind since the beginning of the STEREO mission in 2007 show the notably weak solar wind and interplanetary field described in the literature persists even in the face of the increasing activity of solar cycle 24. These conditions have produced on average low values of geoeffective parameters such as solar wind dynamic pressure and the southward component of the interplanetary field -Bz. In addition, the magnetic disturbances associated with interplanetary coronal mass ejections (ICMEs) have been primarily northward in their leading edges where their fields are compressed by their ambient solar wind interaction. This combination has generally reduced the strengths of storms produced by the ICMEs and stream interaction regions. Interestingly, the rate of CMEs in coronal images has been similar to the previous cycle which had a significantly higher sunspot number and related solar surface field. We summarize how recent conditions on the Sun have modified those that affect the Earth and planets, and the likely trends we may expect.
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    ABSTRACT: We discuss the temporal variations and frequency distributions of solar wind and interplanetary magnetic field parameters during the solar minimum of 2007 – 2009 from measurements returned by the IMPACT and PLASTIC instruments on STEREO-A. We find that the density and total field strength were significantly weaker than in the previous minimum. The Alfvén Mach number was higher than typical. This reflects the weakness of magnetohydrodynamic (MHD) forces, and has a direct effect on the solar wind–magnetosphere interactions. We then discuss two major aspects that this weak solar activity had on the magnetosphere, using data from Wind and ground-based observations: i) the dayside contribution to the cross-polar cap potential (CPCP), and ii) the shapes of the magnetopause and bow shock. For i) we find a low interplanetary electric field of 1.3±0.9 mV m−1 and a CPCP of 37.3±20.2 kV. The auroral activity is closely correlated to the prevalent stream–stream interactions. We suggest that the Alfvén wave trains in the fast streams and Kelvin–Helmholtz instability were the predominant agents mediating the transfer of solar wind momentum and energy to the magnetosphere during this three-year period. For ii) we determine 328 magnetopause and 271 bow shock crossings made by Geotail, Cluster 1, and the THEMIS B and C spacecraft during a three-month interval when the daily averages of the magnetic and kinetic energy densities attained their lowest value during the three years under survey. We use the same numerical approach as in Fairfield’s (J. Geophys. Res. 76, 7600, 1971) empirical model and compare our findings with three magnetopause models. The stand-off distance of the subsolar magnetopause and bow shock were 11.8 R E and 14.35 R E, respectively. When comparing with Fairfield’s (1971) classic result, we find that the subsolar magnetosheath is thinner by ∼1 R E. This is mainly due to the low dynamic pressure which results in a sunward shift of the magnetopause. The magnetopause is more flared than in Fairfield’s model. By contrast the bow shock is less flared, and the latter is the result of weaker MHD forces.
    Solar Physics 01/2012; 281(1). DOI:10.1007/s11207-012-0119-1 · 3.81 Impact Factor
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    ABSTRACT: In this paper we present in situ observations of the heliospheric plasma sheet (HPS) from STEREO-A, Wind, and STEREO-B over four solar rotations in the declining phase of Solar Cycle 23, covering late March through late June 2007. During this time period the three spacecraft were located in the ecliptic plane, and were gradually separating in heliographic longitude from about 3 degrees to 14 degrees. Crossings of the HPS were identified using the following criteria: reversal of the interplanetary magnetic field sector, enhanced proton density, and local minima in both the proton specific entropy argument and in the alpha particle-to-proton number density ratio (N a/N p). Two interplanetary coronal mass ejections (ICMEs) were observed during the third solar rotation of our study period, which disrupted the HPS from its quasi-stationary state. We find differences in the in situ proton parameters at the HPS between the three spacecraft despite temporal separations of less than one day. We attribute these differences to both small separations in heliographic latitude and radial evolution of the solar wind leading to the development of compression regions associated with stream interaction regions (SIRs). We also observed a modest enhancement in the density of iron ions at the HPS.
    Solar Physics 01/2012; 281(1). DOI:10.1007/s11207-012-0156-9 · 3.81 Impact Factor
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    ABSTRACT: Since the launch of the STEREO mission in 2006, there has been renewed interest in the 3D structure of the solar wind, spurred in part by the unusual cycle 23 solar minimum and current solar cycle rise. Of particular significance for this subject has been the ubiquitous occurrence of low latitude coronal holes and coronal pseudo-streamers. These coupled features have been common both because of the relative strength of high order spherical harmonic content of the global coronal field, and the weakness of the field compared to the previous two well-observed cycles. We consider the effects of the low latitude coronal holes and pseudo-streamers on the near-ecliptic solar wind and interplanetary field. In particular, we illustrate how the now common passage of streams with low latitude sources and pseudo-streamer boundaries is changing our traditional perceptions of local solar wind structures.
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    ABSTRACT: STEREO SWEA provides the opportunity to observe solar wind halo electron heat fluxes and strahl over 4pi steradians at locations free of Earth bow shock contamination. After 4+ years of observations, and with the onset of solar cycle 24, we can begin to take a detailed look at the pitch angle distributions related to coronal transients impactng the interplanetary flux budget. Of primary interest are the conditions that lead to hour-scale, strong electron heat fluxes pointed back toward the Sun and counterstreaming or bi-directional electrons. Both of these phenomena have been associated with dynamical reconfigurations of the coronal fields, and in the case of counterstreaming, with the addition of magnetic flux to the heliosphere. We reconsider the different types of behavior known to be present in and around ICMEs from previous studies. Our analyses suggest how ICMEs enable the observed cycle variation in the interplanetary field strength. In particular, we show how counterstreaming can correspond to both the opening and closing of magnetic flux rooted at the Sun when other source(s) of suprathermals are considered.
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    ABSTRACT: 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 ejecta's 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.
    The Astrophysical Journal 07/2011; 737(1):16. DOI:10.1088/0004-637X/737/1/16 · 6.28 Impact Factor