K. Simunac

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

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Publications (39)23.22 Total impact

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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;
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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;
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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.
    AGU Fall Meeting Abstracts. 12/2011;
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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.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: STEREO SWEA (Solar Wind Electron Analyzer) provides the opportunity to observe solar wind halo electron heat fluxes and strahl over 4pi steradians at locations free of Earth bow shock contamination. We have analyzed these measurements together with the magnetic field and plasma parameters to determine their organization with solar wind stream structure during the period between early 2007 and late 2009. This period is characterized by a very low level of solar activity and thus presents an opportunity to diagnose the anomalous features, determining their location and character. This includes heat fluxes that appear to be traveling back toward the Sun, possibly indicating folded interplanetary field lines, interplanetary field loops which may be part of ICMEs, or sources of suprathermal electrons at shocks beyond 1 AU. Our results give a broad view of the issues related to using heat fluxes to interpret interplanetary field topology, even with the benefit of 4pi observations and two spacecraft.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: Discrete wave packets of strong narrow-band ion cyclotron waves are observed ubiquitously in the inner heliosphere from 0.3 to 1 AU, in observations by the high-resolution magnetometers on the STEREO, MESSENGER, Helios, and Venus Express missions. Occasionally, these waves appear quasi-continuously and last for tens of minutes. All waves occur nearly circularly-polarized, with magnetic perturbations transverse to the background field, propagating close to the magnetic field. These waves appear both left-hand and right-hand polarized in the spacecraft frame, consistent with the propagation of left-handed waves moving both toward and away from the Sun in the plasma frame. In the solar wind frame, the waves are left-hand polarized and below the local proton gyro-frequency. The waves are not produced locally as their frequency does not depend on local conditions. The relative amplitudes of the right- and left- handed waves are consistent with the scenario that these waves are generated closer to the Sun and damp slowly while being transported outward by the super-Alfvénic solar wind. The correlation of the wave properties with the plasma and suprathermal ion conditions will be investigated using the STEREO field and plasma data. Preliminary modeling results on the wave generation and propagation will be provided to help understand these waves.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: A corotating interaction region (CIR), formed when a fast stream overtakes a preceding slow stream, is the predominant large-scale solar wind structure during the solar minimum. Using the in situ plasma and magnetic field measurement, we have surveyed more than 100 CIRs at 1 AU during 2007 - 2009. About 30% of them drive interplanetary shocks, sometimes pairs of forward-reverse shocks. Approximately 60% of the CIRs are associated with IMF sector boundary crossings, not much different from the solar-cycle average sector-boundary association rate for CIRs. During the 3 years in the heliocentric orbit, the twin STEREO spacecraft moved gradually apart reaching a longitudinal separation of 120o. Meanwhile, MESSENGER, Venus Express, and Ulysses spacecraft had multiple conjunctions with the 1 AU spacecraft (Wind/ACE, STEREO A/B). From the over 100 CIRs, we have chosen a few events for further study as representative cases. These include examples when the solar background is different, when the CIR features are different, and when the spacecraft are separated in different configurations. We study the coronal and solar wind observations in detail and compare them with the coronal and heliospheric model results obtained from the Community Coordinated Modeling Center (CCMC) and the Predictive Science Inc., to examine the various source regions of the fast/slow wind, and to investigate the CIRs' formation and evolution in the inner heliosphere, including how they drive shocks and interact with the IMF sector boundary.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: We present in situ observations of the heliospheric plasma sheet (HPS) from 3 observatories (STEREO-A, STEREO-B, and WIND) over 4 solar rotations: Carrington rotations (CR) 2054 through 2057. At least one interplanetary coronal mass ejection (ICME) occurred during CR 2056, which leads to reorganization of the HPS from its previous quasi-stationary structure. The HPS crossings were identified using criteria including reversal of the interplanetary magnetic field, increased proton density, and local minima in proton entropy and in the alpha to proton density ratio. We also find an enhancement of the average iron charge state at the HPS.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: We investigate the heliospheric current sheet (HCS) crossing events and the related heliospheric plasma sheet (HPS) on both STEREO spacecraft from Mar, 1, 2008 to Dec, 31, 2008. Observed plasma sheets are categorized into two types based on their relative position to the current sheets. Type I plasma sheets straddle the current sheets, and type II plasma sheets are located on one side of the current sheets. The electron heat flux dropouts (HFD) are also documented for each type of plasma sheets. Initially, the investigation was limited to 39 ideal HCS crossings. Among the initial 39 HCS crossings in our study, 4 have no HPS, 21 have a type I HPS, and 13 a Type II HPS. Most of the Type II HCSs don't show a HFD, but a large portion of type I HPSs show a HFD. Later, the study is generalized to all HCS events for which we can determine the actual time and properties of the HPS. This conclusion still holds when all the identifiable HPS are included in the study. Schematic plots summing the different magnetic field configurations are presented, and the potential origin of plasmas forming the two types of HPS is discussed.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: The origin and evolution of the solar wind from coronal holes is studied by characterizing the physical properties of the solar wind plasma (temperature, density, outflow velocity, and composition) with multi-spacecraft and ground-based observations. PFSS modeling is also used to confirm interpretation of the source regions and in wind-stream boundary mapping. We discuss the results for the fast solar wind from polar and low-latitude coronal-hole wind streams. We also compare the characteristics of these wind streams with results from the previous solar minimum.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: We present solar wind trends observed by STEREO-A PLASTIC during Carrington Rotations 2053-2090 (February 2007 to December 2009) in relation to the current solar cycle conditions. The solar wind speed, mass number, and iron average charge state all showed a decline in the latter half of the survey period, during the time of minimal sunspot area, a retreat of the polar coronal holes (but with occasional isolated coronal holes) and a flattening of the neutral line.
    06/2010;
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    ABSTRACT: Previous work has shown that solar wind suprathermal electrons can display a number of features in terms of their anisotropy. Of importance is the occurrence of counter-streaming electron patterns, i.e., with "beams" both parallel and anti-parallel to the local magnetic field, which is believed to shed light on the heliospheric magnetic field topology. In the present study, we use STEREO data to obtain the statistical properties of counter-streaming suprathermal electrons (CSEs) in the vicinity of corotating interaction regions (CIRs) during the period March–December 2007. Because this period corresponds to a minimum of solar activity, the results are unrelated to the sampling of large-scale coronal mass ejections, which can lead to CSE owing to their closed magnetic field topology. The present study statistically confirms that CSEs are primarily the result of suprathermal electron leakage from the compressed CIR into the upstream regions with the combined occurrence of halo depletion at 90° pitch angle. The occurrence rate of CSE is found to be about 15–20% on average during the period analyzed (depending on the criteria used), but superposed epoch analysis demonstrates that CSEs are preferentially observed both before and after the passage of the stream interface (with peak occurrence rate >35% in the trailing high speed stream), as well as both inside and outside CIRs. The results quantitatively show that CSEs are common in the solar wind during solar minimum, but yet they suggest that such distributions would be much more common if pitch angle scattering were absent. We further argue that (1) the formation of shocks contributes to the occurrence of enhanced counter-streaming sunward-directed fluxes, but does not appear to be a necessary condition, and (2) that the presence of small-scale transients with closed-field topologies likely also contributes to the occurrence of counter-streaming patterns, but only in the slow solar wind prior to CIRs.
    Annales Geophysicae 01/2010; · 1.52 Impact Factor
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    ABSTRACT: In this paper we study the occurrence rate and solar origin of interplanetary coronal mass ejections (ICMEs) using data from the two Solar TErrestrial RElation Observatory (STEREO) and the Wind spacecraft. We perform a statistical survey of ICMEs during the late declining phase of solar cycle 23. Observations by multiple, well-separated spacecraft show that even at the time of extremely weak solar activity a considerable number of ICMEs were present in the interplanetary medium. Soon after the beginning of the STEREO science mission in January 2007 the number of ICMEs declined to less than one ICME per month, but in late 2008 the ICME rate clearly increased at each spacecraft although no apparent increase in the number of coronal mass ejections (CMEs) occurred. We suggest that the near-ecliptic ICME rate can increase due to CMEs that have been guided towards the equator from their high-latitude source regions by the magnetic fields in the polar coronal holes. We consider two case studies to highlight the effects of the polar magnetic fields and CME deflection taking advantage of STEREO observations when the two spacecraft were in the quadrature configuration (i.e. separated by about 90 degrees). We study in detail the solar and interplanetary consequences of two CMEs that both originated from high-latitude source regions on 2 November 2008. The first CME was slow (radial speed 298 km/s) and associated with a huge polar crown prominence eruption. The CME was guided by polar coronal hole fields to the equator and it produced a clear flux rope ICME in the near-ecliptic solar wind. The second CME (radial speed 438 km/s) originated from an active region 11007 at latitude 35° N. This CME propagated clearly north of the first CME and no interplanetary consequences were identified. The two case studies suggest that slow and elongated CMEs have difficulties overcoming the straining effect of the overlying field and as a consequence they are guided by the polar coronal fields and cause in-situ effects close to the ecliptic plane. The 3-D propagation directions and CME widths obtained by using the forward modelling technique were consistent with the solar and in-situ observations.
    Annales Geophysicae 01/2009; · 1.52 Impact Factor
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    ABSTRACT: The two STEREO spacecraft with nearly identical instrumentation were launched near solar activity minimum and they separate by about 45° per year, providing a unique tool to study the temporal evolution of the solar wind. We analyze the solar wind bulk velocity measured by the two PLASTIC plasma instruments onboard the two STEREO spacecraft. During the first half year of our measurements (March – August 2007) we find the typical alternating slow and fast solar wind stream pattern expected at solar minimum. To evaluate the temporal evolution of the solar wind bulk velocity we exclude the spatial variations and calculate the correlation between the solar wind bulk velocity measured by the two space-craft. We account for the different spacecraft positions in radial distance and longitude by calculating the corresponding time lag. After adjusting for this time lag we compare the solar wind bulk velocity measurements at the two spacecraft and calculate the correlation between the two time-shifted datasets. We show how this correlation decreases as the time difference 366 A. Opitz et al. between two corresponding measurements increases. As a result, the characteristic temporal changes in the solar wind bulk velocity can be inferred. The obtained correlation is 0.95 for a time lag of 0.5 days and 0.85 for 2 days.
    Solar Physics 01/2009; 256:365-377. · 3.26 Impact Factor
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    ABSTRACT: We report the in situ observations of the ion cyclotron waves in the solar wind near 0.3, 0.7, and 1 AU, mostly based on the high-resolution magnetometer data from Helios, Venus Express, and STEREO spacecraft. These waves are circularly-polarized transverse waves, propagating nearly parallel to the magnetic field, and below the local proton gyro-frequency in the solar wind frame. Since these waves often occur when the field is close to the radial direction, the angles between the magnetic field and the solar wind velocity are small, and the waves are unlikely to be generated by pickup ions. Although the waves appear both left-handed and right-handed in the spacecraft frame, this can be explained by Doppler shift. In our scenario, they are all left-handed polarized waves generated closer to the Sun, and are carried out by the super-Alfvénic solar wind. In the spacecraft frame, the anti-sunward waves will be shifted to higher frequencies and the sunward waves will be shifted to lower frequencies with their polarization reversed. We analyze the wave properties of each event to obtain the statistics of the wave power and wave frequency at each heliocentric distance. By comparing these wave properties at different distances, we hope to constrain further their evolution and their generation mechanism.
    01/2009;
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    ABSTRACT: We have analyzed solar wind oxygen in the data from the STEREO Plasma and Supra-thermal Ion Composition Experiment (PLASTIC). For this initial study we concentrate on the slow solar wind where the ion composition is stable, different ion species have nearly the same bulk speed, and the kinetic temperature is usually low. The mass of the detected ions is determined when the ions have both a valid time-of-flight and a residual energy measured by a Solid State Detector (SSD). The bulk speed, thermal speed and flow angles of O6+ are then calculated using the electrostatic analyzer and position data. The STEREO data are compared to similar measurement on ACE/SWICS.
    AGU Fall Meeting Abstracts. 12/2008;
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    ABSTRACT: Since the late 1960's, suprathermal and energetic ion events with energies ranging from just above the solar wind energies up to 2MeV and lasting for several minutes to hours, have been detected upstream of the Earth. Possible sources of these ions include magnetospheric ions, solar wind ions accelerated between the Earth's bow shock and hydromagnetic waves to energies just above the solar wind energies, and remnant ions from heliospheric processes (such as Solar Energetic Particle (SEP) events or Corotating Interaction Regions (CIRs)). The unique orbits of both STEREO spacecraft, STEREO-A (STA) drifting ahead in Earth's orbit and STEREO-B (STB) lagging behind in Earth's orbit, allow for analysis of upstream events in these unexamined regions. Using both the PLASTIC and IMPACT instruments on board STA/B we can examine protons in the energy range of solar wind energies up to 80keV, their spatial distribution, and determine if the spacecraft is magnetically connected to the Earth's bow shock. Suprathermal events observed by STEREO/PLASTIC during solar minimum conditions are examined for possible upstream events using anisotropy measurements, velocity dispersion, magnetic connection to the bow shock, and frequency of events as a function of time and distance.
    AGU Fall Meeting Abstracts. 12/2008;
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    ABSTRACT: Although the quiet activity period surrounding the current solar minimum has prevailed since the launch of STEREO in October 2006, there have been at least 9 clear in-situ detections of ICMEs (Interplanetary Coronal Mass Ejections) by one or more spacecraft during the time the imagers were also operating. These observations provide unusually complete data sets for evaluating helio-longitude extent of the ICMEs and for identifying the probable solar cause(s) of the events. In this poster we present information on these ICMEs from the IMPACT and PLASTIC and ACE in-situ investigations, together with solar images from STEREO and SOHO that seem to capture the causative activity at the Sun. We find that even though the Sun was very quiet in '07-'08, with few active regions visible in GONG and SOHO magnetograms, there were numerous CME candidates that erupted through the near-equatorial helmet streamers. Most events could be identified with EUV disk activity as well as a coronagraph CME, even if the associated active region was very small or weak. Old cycle active regions, new and decayed, continued to maintain a warp in the large-scale helmet streamer belt that was a frequent site of the eruptions. However, the warp in the streamer belt may simply indicate that the active region(s) present is(are) sufficiently strong to affect the large scale quiet coronal field structure. Overall we see no gross differences between the solar activity and ICME causes during this and the previous solar activity minimum, when the Streamer belt was less warped due to the existence of stronger solar polar fields.
    AGU Fall Meeting Abstracts. 12/2008;
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    ABSTRACT: We explore sources of apparent time-dependence of meso-scale structures (those lasting two to three days and less) in the ambient solar wind through analysis of measurements from STEREO-A, -B, and L1 spacecraft (WIND, ACE, and SOHO). In early 2008, stable corotating interaction regions and high-speed streams provided excellent boundaries and features for co-registering the large-scale, corotating solar wind observed by several heliospheric spacecraft separated in solar orbital phase near 1 AU. During this period, STEREO-B (located 23 degrees behind the Earth in heliographic longitude) first observed the large-scale corotating stream structures, followed by the WIND, ACE, and SOHO spacecraft at Earth, then finally by STEREO-A (located 22 degrees ahead of the Earth in heliographic longitude). Conspicuous similarities in the macro-scale solar wind flow dominate the comparison between spacecraft observations and permit us to time-adjust the observed flow features reasonably well by assuming a simple corotating solar wind source. While the co-registered, large-scale solar wind structure agrees well, mesoscale flow features can exhibit large measured differences at the various spacecraft. We focus on one such interesting feature which exhibits apparent time dependence. Though this few-day-long, significant flow speed event is observed by the PLASTIC experiments on both STEREO-A and STEREO-B, it is not seen at the L1 spacecraft which the STEREO spacecraft bracket in space and time. We explore potential sources of the apparent time dependence of this meso-scale feature. Latitudinal differences in the multipoint measurements is one source that could account for the apparent mesoscale flow structure variability. We also explore explicit time variation of the solar wind's source, by analyzing relevant coronal holes observed simultaneously by the STEREO spacecraft imagers. This event and analysis underscores that multipoint heliospheric observations and analysis reveals the existence of mesoscale structure in the solar wind and can be used to constrain its possible source(s).
    AGU Fall Meeting Abstracts. 12/2008;
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    ABSTRACT: Pickup ions seem to be a perfect tracer of interplanetary discontinuities in the heliosphere and they provide important information on acceleration processes at these structures and in the turbulent solar wind (i.e. suprathermal tails). Studies of pickup ions using AMPTE, Ulysses, SOHO, Wind and ACE demonstrated that pickup ion fluxes and the shape of their distributions can vary substantially on time scales from less than one hour to many days. These variations have been attributed to changes in the interplanetary magnetic field (IMF) direction and strength in the sense of incomplete pickup and/or density compressions and decompressions. For instance, at CIRs one observes the most intense and most prolonged enhancements of energetic helium pickup ions. At present, the vast majority of the observed temporal variations remain unexplained. Furthermore, spatial variations of pickup ion distributions could not be studied with single spacecraft observation. Simultaneous observations of pickup ion distributions with the PLASTIC instrument on STEREO A and B now provide the opportunity to follow pickup ion variations on spatial scales from a few 106 km to 108 km. In the early mission phase STEREO A and B were often along the same magnetic field flux tubes. This allows us to study temporal effects. With increasing spacecraft separation spatial effects can be studied. In this presentation we will show STEREO observations of helium pickup ion spectra and fluxes for 2007/8 in their dependence on solar wind density, speed and flux as well IMF direction and strength on both spacecraft. We then determined whether the observed variations are mainly correlated features that are associated with spatial structures passing the STEREO spacecraft at different times (such as CIRs or the focusing cone), or whether they have a substantial uncorrelated component indicative of temporal variations.
    AGU Fall Meeting Abstracts. 11/2008; -1:1605.