Bidirectional Electron Heat Flux Research Articles

Interaction of the bow shock with a tangential discontinuity and solar wind density decrease: Observations of predicted fast mode waves and magnetosheath merging

Shortly after 0600 UT on 7 April 2000 a tangential discontinuity (TD) in the solar wind passed the Advanced Composition Explorer satellite (ACE). It was characterized by a rotation of the interplanetary magnetic field (IMF) by ∼145° and more than a factor‐of‐2 decrease in the plasma density. About 50 min later, Polar encountered more complex manifestations of the discontinuity near noon in the magnetosheath outside the Northern Hemisphere cusp. On the basis of Polar observations, theoretical modeling, and MHD simulations we interpret the event as demonstrating that (1) a fast mode rarefaction wave was generated during the TD‐bow shock interaction, (2) the fast wave carried a significant fraction of the density change to the magnetopause while the remainder stayed with the transmitted discontinuity, and (3) magnetic merging occurred between IMF field lines within the magnetosheath on opposite sides of the discontinuity's surface as it approached the magnetopause. Before the discontinuity passed the spacecraft, Polar detected ions accelerated antiparallel to B in the fast wave and perpendicular to B in a weak slow mode structure located adjacent to and just downstream of the fast wave. The antiparallel accelerated ions in the fast wave had no measurable ion‐velocity dispersion signature, placing their source a few RE equatorward of Polar. Simulation results, a Walén test, detections of wave Poynting flux parallel to B, bidirectional electron heat flux, and ion velocity enhancements all indicate that the three ion bursts associated with the passage of the discontinuity were signatures of time‐dependent, magnetic merging events within the magnetosheath.

A magnetic polarity and chirality analysis of ISEE 3 interplanetary magnetic clouds

We have used solar wind electron heat fluxes to determine the magnetic polarities of the 14 magnetic clouds observed with the ISEE 3 spacecraft in 1978–1982. The clouds have been modeled as single magnetic flux ropes, and it is generally assumed that they are magnetically closed structures with each end of the flux rope connected to the Sun. In that case, the magnetic polarity of each cloud should be unchanged during the passage of ISEE 3 through the cloud. We use the dominant heat flux in bidirectional electron heat fluxes to determine the cloud polarities and find that the polarity changes in at least 2 and possibly 6 of the 14 clouds, meaning that those clouds cannot fit the popular model of a single flux rope.We have also examined the large‐scale fields with bidirectional electron fluxes that follow some magnetic clouds. We recently conjectured that these following fields were the trailing legs of flux rope loops. For that picture to be valid, the clouds and their trailing fields must have opposite polarities and matching magnetic chiralities. We use a simple scheme to test for magnetic chiralities of those clouds and their trailing fields and find that only 1 of 8 may have the flux rope loop geometry.

Ulysses observation of a noncoronal mass ejection flux rope: Evidence of interplanetary magnetic reconnection

A well‐defined, small‐scale (≈0.05 AU) magnetic flux rope was observed by Ulysses at about 5 AU in close proximity to a heat flux dropout (HFD) at the heliospheric current sheet (HCS). This magnetic flux rope is characterized by a rotation of the field in the plane approximately perpendicular to the ecliptic (and containing the Sun and the spacecraft) and with a magnetic field maximum centered near the inflection point of the bipolar signature. The edges of the flux rope are well defined by diamagnetic field minima. A bidirectional electron heat flux signature is coincident with the magnetic flux rope structure. The event occurred during a time of slightly increasing solar wind speed, suggesting that the field and plasma were locally compressed. Unlike most coronal mass ejections/magnetic clouds, this event is characterized by high proton temperatures and densities, high plasma beta, no significant alpha particle abundance increase, and a small radial size. We interpret these observations in terms of multiple magnetic reconnection of previously open field lines in interplanetary space at the HCS. Such reconnection produces a U‐shaped structure entirely disconnected from the Sun (which we associate with the HFD), a closed magnetic flux rope (which we associate with the counterstreaming electron event), and a closed magnetic loop or tongue connected back to the Sun at both ends. These observations suggest that magnetic reconnection, and its changes to magnetic field topology, can occur well beyond the solar corona in interplanetary space.

A Survey of Bidirectional >= 1 MeV Ion Flows during the HELIOS 1 and HELIOS 2 Missions: Observations from the Goddard Space Flight Center Instruments

view Abstract Citations (13) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey of Bidirectional >= 1 MeV Ion Flows during the HELIOS 1 and HELIOS 2 Missions: Observations from the Goddard Space Flight Center Instruments Richardson, I. G. Abstract A survey is made of 1.1-2.1 MeV and 3-21 MeV bidirectional ion flows (BIFs) in the solar wind at 0.3-1.0 AU observed by the Goddard Space Flight Center instruments on the Helios 1 and Helios 2 spacecraft during the spacecraft missions in 1974-1984 and 1976-1980, respectively. Some 774 events have been identified at Helios 1, and 223 at Helios 2, with mean durations of ∼2.5 hr at each spacecraft. BIFs were observed for ∼1%-3% of the observation time during solar minimum conditions in 1975-1977 and for ∼10%-14% in 1980-1981 around solar maximum. These occurrence rates are similar to those of ∼1 MeV BIFs observed at the IMP 8 and ISEE 3 spacecraft, and of bidirectional electron heat fluxes at ISEE 3. Solar wind plasma parameters during BIFs were similar to those found generally during the Helios missions, though BIFs have a greater tendency to be associated with cool, low-β plasma, moderately enhanced and low-variance magnetic fields, and both less than 300 km s-1 and greater than 750 km s-1 solar wind. Solar wind features associated with the greater than 4 hr duration BIFs include clear plasma shock-driver signatures: 36% of the events; postshock plasma, not associated with shock-driver signatures: 13 %; noncompressive density enhancements: ∼22% (half associated with plasma proton temperature depressions); high-speed streams not associated with shocks: ∼10%; corotating interaction regions: ∼5%; immediately upstream of shocks: ∼5%. Around 8% were associated with other solar wind structures, while ∼4% showed no associated structure. BIFs were present in several of the shock drivers associated with coronal mass ejections (CMEs) observed by the Solwind coronagraph during intervals when Helios 1 was off the limbs of the Sun, showing directly the association of some BIFs with CMEs. Publication: The Astrophysical Journal Pub Date: January 1994 DOI: 10.1086/173616 Bibcode: 1994ApJ...420..926R Keywords: SPACE VEHICLES; SUN: PARTICLE EMISSION; SUN: SOLAR WIND full text sources ADS |

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

While relatively common in the outer heliosphere, forward‐reverse shock pairs occur rarely in the solar wind at 1 AU. Using the combined solar wind plasma and magnetic field data from ISEE 3 for the interval from August 1978 through February 1980 we have identified several examples of such shock pairs at 1 AU. Two of these shock pair events were clearly associated with transient disturbances rather than with corotating streams. Strong bidirectional electron heat flux events, which we identify as the coronal mass ejecta (CME) driving the shocks, were detected during both of these disturbances. In the stronger of the two disturbances the reverse shock was found within the CME and was separated from the forward shock by ∼0.2 AU; the pressure was nearly constant between the two shocks. On the other hand, in the weaker of the two disturbances the reverse shock had propagated entirely through the CME and trailed the forward shock by ∼0.3–0.4 AU; the pressure declined substantially and monotonically between the two shocks. The overall disturbance profiles for these two events compare favorably with the results of previously published numerical simulations. On the basis of those simulations we suggest that these two events are prototypes of “driven” and “blast” disturbances, wherein the time scale of the initial velocity perturbation near the Sun is, or is not, short compared to the transit time to 1 AU. Our observations support the notion that reverse shocks in transient disturbances are most likely to be found near the central axes of such disturbances.

Interplanetary magnetic field orientations associated with bidirectional electron heat fluxes detected at ISEE 3

A statistical survey of interplanetary magnetic field orientations associated with bidirectional electron heat fluxes observed at ISEE 3 in orbit about the Sunward Lagrange point indicates that magnetic connection of the spacecraft to the Earth's bow shock was frequently the source of the bidirectionality. When the interplanetary magnetic field was oriented within 5° of the Earth‐spacecraft line, backstreaming electrons from the bow shock were clearly observed approximately 18% of the time, and connections apparently occurred for angles as large as ∼30°–35°.

The eastward deflection of fast coronal mass ejecta in interplanetary space

Previous work has shown that a bidirectional solar wind electron heat flux is one of the more prominent signatures of a coronal mass ejection event in the solar wind at 1 AU. Using ISEE 3 solar wind electron measurements obtained during 1978 and 1979, we have used this signature to identify the fast coronal mass ejecta driving 19 interplanetary shocks. In 17 of the 19 shock events an eastward deflection of the ejection plasma (apparent arrival from west of the sun) was observed. The average eastward deflection for all of the events was ∼3°, corresponding to a typical transverse velocity of ∼25 km/s. Usually an oppositely directed (i.e., westward) flow deflection of comparable magnitude was observed within the compressed ambient plasma ahead of the ejecta. The sense of these deflections—first westward within the compressed ambient plasma and then eastward within the ejecta—is the same as is observed near the leading edges of quasi‐stationary, corotating high speed streams. However, in contrast to the case of corotating streams, the west‐east flow reversal does not coincide with a local maximum in static pressure. No consistent flow deflection pattern perpendicular to the ecliptic has been observed for these events, and no similar deflections have been observed for ejecta (bidirectional electron heat flux events) traveling at the same speed as or slower than the ambient plasma ahead. We suggest that the preferred eastward deflection of these fast coronal mass ejecta is a consequence of solar rotation and the spiral geometry of the ambient solar wind.

Bidirectional solar wind electron heat flux events

Normally the ≳ 80‐eV electrons which carry the solar wind electron heat flux are collimated along the interplanetary magnetic field (IMF) in the direction pointing outward away from the sun. Occasionally, however, collimated fluxes of ≳ 80‐eV electrons are observed traveling both parallel and antiparallel to the IMF. Here we present the results of a survey of such bidirectional electron heat flux events as observed with the plasma and magnetic field experiments aboard ISEE 3 at times when the spacecraft was not magnetically connected to the earth's bow shock. The onset of a bidirectional electron heat flux at ISEE 3 usually signals spacecraft entry into a distinct solar wind plasma and field entity, most often characterized by anomalously low proton and electron temperatures, a strong, smoothly varying magnetic field, a low plasma beta, and a high total pressure. Significant field rotations often occur at the beginning and/or end of bidirectional heat flux events, and, at times, the large field rotations characteristic of “magnetic clouds” are present. Approximately half of all bidirectional heat flux events are associated with and follow interplanetary shocks, while the other events have no obvious shock associations. When shock associated, the delay from shock passage typically is ∼13 hours, corresponding to a radial separation of ∼0.16 AU. Independent of any shock association, bidirectional heat flux events typically are ∼0.13 AU thick in the radial direction, although considerable variability is evident from one event to another. Near solar activity maximum, bidirectional heat flux events occurred at a rate of ∼3 per month, and the solar wind electron heat flux was bidirectional ∼5% of the time. Bidirectional heat flux events often contain strong out‐of‐the‐ecliptic field components and thus can be effective in producing geomagnetic disturbances. This is particularly true for shock‐associated events where the intrinsically strong fields in the leading portions of the events are amplified by compression in transit from the sun and where strong out‐of‐the‐ecliptic field components resulting from compression and draping of the ambient field are often present within the shocked plasma immediately ahead. Consistent with previous work we interpret the bidirectional heat flux as evidence for a closed field topology in interplanetary space. Further, we suggest that these events are one of the more prominent signatures of coronal mass ejection events in the solar wind at 1 AU.