A. Opitz

Multispacecraft observation of magnetic cloud erosion by magnetic reconnection during propagation

During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19-20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large-scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12-0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo-effectiveness. ©2012. American Geophysical Union. All Rights Reserved.

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 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.

Statistical features of the global polarity reversal of the Venusian induced magnetosphere in response to the polarity change in interplanetary magnetic field

In this study we present the first statistical analysis on the effects of heliospheric current sheet crossings on the induced magnetosphere of Venus. These events are of particular interest because they lead to the reconfiguration of the induced magnetosphere with opposite polarity. We use a statistical approach based on 117 orbit pairs, and we study the spatial distribution of the heavy ion flux measurements in the plasma environment of Venus. The average and median heavy ion flux measurements are compared before and after the polarity reversal events. The results show that after the events the average and median heavy ion fluxes in the magnetotail are reduced by the factors of 0.75 ± 0.09 and 0.52, respectively. We find that even if a passage of a current sheet is a short time scale event lasting about 10 min, its effect on the near-Venus plasma environment lasts for a few hours. We conclude that the observations show similarities to the previous comet studies and the polarity reversal of the induced magnetosphere might be accompanied with dayside reconnection and magnetic disconnection of the plasma tail from the planetary ionosphere.

Space weather effects on the bow shock, the magnetic barrier, and the ion composition boundary at Venus

We present a statistical study on the interaction between interplanetary coronal mass ejections (ICMEs) and the induced magnetosphere of Venus when the peak magnetic field of the magnetic barrier was anomalously large (>65 nT). Based on the entire available Venus Express data set from April 2006 to October 2014, we selected 42 events and analyzed the solar wind parameters, the position of the bow shock, the size and plasma properties of the magnetic barrier, and the position of the ion composition boundary (ICB). It was found that the investigated ICMEs can be characterized with interplanetary shocks and unusually large tangential magnetic fields with respect to the Venus-Sun line. In most of the cases the position of the bow shock was not affected by the ICME. In a few cases the interaction between magnetic clouds and the induced magnetosphere of Venus was observed. During these events the small magnetosonic Mach numbers inside magnetic clouds caused the bow shock to appear at anomalously large distances from the planet. The positions of the upper and lower boundaries of the magnetic barrier were not affected by the ICMEs. The position of the ICB on the nightside was found closer to the planet during ICME passages which is attributed to the increased solar wind dynamic pressure.

Solar wind control of the terrestrial magnetotail as seen by STEREO

At the beginning of 2007 the twin STEREO spacecraft provided a unique opportunity to study the global solar wind control of the terrestrial magnetotail under typical solar activity minimum conditions. The STEREO-B (STB) spacecraft flew in the vicinity of the far terrestrial magnetotail, while the STEREO-A (STA) spacecraft was located in front of the Earth performing measurements in the undisturbed solar wind. In February, the STB spacecraft was located in the magnetosheath most of the time but experienced several incursions into the distant magnetotail. Comparison of STA and STB observations determines unambiguously whether solar wind events such as energetic particle enhancements observed by STB are of pure solar origin or due to the influence of the terrestrial magnetosphere. During this time period in 2007, there were solar minimum conditions with alternating fast and slow solar wind streams that formed corotating interaction regions, which were the dominating source of magnetospheric disturbances encountering the Earth almost every week. Under these conditions, STB experienced multiple bow shock and magnetopause crossings due to the induced highly dynamic behavior of the terrestrial magnetotail and detected bursts of tailward directed energetic ions in the range of 110–2200 keV accompanied by suprathermal electrons of ~700–1500 eV, which were not seen in the undisturbed solar wind by STA. The corotating interaction regions triggered these energetic particle enhancements, and we demonstrate their magnetosphere-related origin. Even after leaving the magnetosheath in March 2007, STB continued to observe antisunward directed energetic ion bursts until May up to a distance of ~ 800 RE behind Earth, the largest distance to which solar wind and magnetospheric interaction has been observed. These results show that Earth is a very significant source of energetic particles in its local interplanetary environment.

Hot flow anomaly remnant in the far geotail

A hot flow anomaly (HFA) like event was observed by the Solar TErrestrial RElations Observatory (STEREO) in the night side magnetosheath in the far tail in February-March 2007. The magnetic signature of the tangential discontinuity was visible, but the resolution of the plasma ion data is not sufficient for our analysis, so a method is given to identify HFAs without solar wind velocity measurements. The event observed in the night side magnetosheath in the far tail might be the remnant of an HFA event, a not-so-active current sheet. This observation suggests that the lifetime of the HFAs might be several 10 minutes, much longer than the expected several minutes.

Multipoint connectivity analysis of the May 2007 solar energetic particle events

In May of 2007, the STEREO Ahead and Behind spacecraft, along with the ACE spacecraft situated between the two STEREO spacecraft, observed two small solar energetic particle (SEP) events. STEREO-A and -B observed nearly identical time profiles in the 19 May event, but in the 23 May event, the protons arrived significantly earlier at STEREO-A than at STEREO-B and the time-intensity profiles were markedly different. We present SEP anisotropy, suprathermal electron pitch angle and solar wind data to demonstrate distortion in the magnetic field topology produced by the passage of multiple interplanetary coronal mass ejections on 22 and 23 May, causing the two spacecraft to magnetically connect to different points back at the Sun. This pair of events illustrates the power of multipoint observations in detailed interpretation of complex events, since only a small shift in observer location results in different magnetic field line connections and different SEP time-intensity profiles.