Hot Flow Anomalies Research Articles

Distributions of suprathermal ions near hot flow anomalies observed by RAPID aboard Cluster

A series of events having signatures of hot flow anomalies (HFA) observed by the magnetic field, plasma, and energetic particle instruments aboard the Cluster spacecraft upstream of the Earth’s bow shock in February–April 2003 are investigated. During this period, the separation of the four satellites (up to about 10,000 km) allowed determining the velocity of moving interplanetary plasma structures, which intersect the bow shock. About 50 candidates for HFA events were found and some statistical aspects are discussed. Two HFA events are analyzed in detail: one on 16 February 2003 at 10:48 UT with marked energetic ion signature and another on 7 March 2003 at 10:15 UT with a smaller increase. Both occurred in the vicinity of the bow shock, the plasma speed decreased, its flow deflected, its temperature increased, whereas a magnetic structure with rapidly changing field direction passed by and finally reached the bow shock. Both electrons and ions are accelerated; elevated fluxes of 28–68 keV ions appeared before and lasted longer than the HFA event as observed by all four RAPID instruments. The directional and pitch angle distributions of low-energy ions transformed into the plasma frame are discussed.

A study of particle flows in hot flow anomalies

Hot flow anomalies (HFAs) are distinct disturbances of the solar wind flow observed often in the bow shock vicinity. Both experiment and theory suggest that these disturbances are created at the intersection of the bow shock and tangential discontinuity (TD) of the interplanetary magnetic field (IMF). The discontinuity is swept along the bow shock by the solar wind flow and thus the resulting HFA containing hot, tenuous, and deflected plasma can be found not only in the subsolar region but also on the flanks. We have identified three HFAs in measurements of the INTERBALL-1 and MAGION-4 spacecraft. The first one was observed in the solar wind and two others created an interface between the solar wind and magnetosheath. We have analyzed the flow directions inside these HFAs in order to find a relationship between particle flows and orientations of original IMF discontinuities. Our results show that, beside the dominant plasma motion connected with the sweeping of the discontinuity, a significant velocity component directed along the discontinuity can be found. This suggests a highly elongated shape of the HFA cavity. The correlation length of particular HFA features (amplitude of bounding density enhancements, their duration, etc.) may be rather short; it is of the order of 1 R E in our case. We also suggest that HFA, which extends into the magnetosheath, modifies locally downstream the Mach number and causes a local displacement of the bow shock.

Hot Dry Rock Geothermal Exploration in Australia

Hot dry rock (HDR) geothermal energy is obtained by circulating water between injection and production wells through hot subsurface rocks. The recovered hot water should be around 250°C for efficient electricity generation. South Australia has become a focus for HDR developments due to its exceptionally hot subsurface rocks. Previous HDR projects have focused on areas of known high geothermal gradient, based, for example, on experience from petroleum wells, e.g. the European Soultz-sous-Forêts site and Geodynamics’ Habanero-1 well in the Cooper Basin. An alternative strategy is to explore for the highest geothermal gradients closest to electricity markets. Petratherm Ltd. holds geothermal exploration licences within the uniquely hot South Australian Heat Flow Anomaly and will target buried thermally anomalous granites and radiogenic iron oxides therein. Thermal modelling indicates that temperatures of 250°C may be attained at depths <4 km within the licences. The thermal conductivity of the cover rocks is as important a factor as the heat-generating potential of the basement in generating exceptionally high geothermal gradients.

Cluster observations of hot flow anomalies

On 2 April 2002, Cluster entered the solar wind at high northern latitudes and observed a series of disturbances upstream of the bow shock. We suggest that these were signatures of a sequence of hot flow anomalies (HFAs), observed at different stages of development. We estimate the orientation and motion of the HFAs and structures within them using multispacecraft data and examine the variation of plasma flow in the context of the orientation of the underlying discontinuities. We demonstrate that the motional electric field at each disturbance was in a direction such that it would act to focus ions reflected from the bow shock toward the discontinuity, consistent with current understanding of HFA formation. The first disturbance consisted of hot plasma flowing along the estimated discontinuity plane during the core of the event, together with a weakened solar wind beam. Toward the edge of the event, compressed solar wind plasma flowed approximately perpendicular to the discontinuity plane, consistent with expansion of the hot plasma. We suggest that the presence of multiple ion distributions signifies that the HFA was at an early stage of its evolution. A second HFA, which had a more complex signature, contained a single ion distribution within the event core, which is the more typical signature of an HFA. It also contained a large velocity deflection, consistent with expansion of the plasma perpendicular to the discontinuity plane. The final HFA event we present is associated with a shock encounter, demonstrating the level of complexity that can be associated with the bow shock response to changing upstream conditions.

A detailed description of the solar wind triggers of two dayside transients: Events of 25 July 1997

Traveling Convection Vortices (TCVs), a specific type of dayside transient event, offer a good opportunity to study the modes of interaction between the solar wind, bow shock and the magnetosphere/ionosphere system. We study here in detail the solar wind triggers of two TCV events that occurred at 1500 UT and 1835 UT on 25 July 1997. During these two events there was a fortuitous conjunction of four solar wind monitors near the Earth's magnetosphere. We were able to identify the exact solar wind discontinuity that triggered each ground TCV. We found that the 1500 UT TCV was triggered by the density (and thus dynamic pressure) enhancement accompanying a tangential discontinuity. We were able to determine the orientation of the discontinuity plane and thus predict, in good agreement with observations, the propagation of the discontinuity between the four spacecraft as well as the propagation of the transient in the magnetosphere (geosynchronous) and ionosphere. The ground transient was generated in the early afternoon local time and then propagated westward, first toward local noon and then away from noon toward dawn. The 1835 UT TCV, which was stronger, was triggered by a more complicated solar wind discontinuity that exhibited significant spatial structure and an unusual propagation pattern. A detailed analysis of the orientation of the discontinuity fronts explained the propagation of the discontinuity in the solar wind. Even though the discontinuity did not carry a significant dynamic pressure enhancement, it had the properties required to generate a hot flow anomaly (HFA) or a foreshock cavity at the bow shock. The corresponding pressure reduction in the magnetosheath interacted with the magnetosphere to generate the transient near local noon that propagates eastward toward dusk. Geosynchronous observations confirm these conclusions. To strengthen our conclusions, we compared the discontinuity that triggered the ground TCV event with four other discontinuities that occurred in the same 2‐hour window. We found that none of the other four discontinuities carried significant dynamic pressure enhancements or had properties required to create a HFA at the bow shock. They did not trigger detectable transients on the ground. It thus seems that TCVs can be triggered by solar wind discontinuities that either carry dynamic pressure enhancements (or alternatively reductions) or by discontinuities that create HFAs or foreshock cavities at the bow shock.

Statistical identification of solar wind origins of magnetic impulse events

We have compiled a complete list of magnetic impulse events (MIEs) during the 8‐year period of 1995–2002 covering solar minimum to solar maximum using fluxgate magnetometer data obtained at the South Pole. Wavelet analysis enables us to detect 825 distinct MIEs automatically with high confidence. Interplanetary magnetic field (IMF) discontinuities are also detected automatically for the same period from Wind and ACE satellite data. From an examination and comparison of the two lists, we find that monthly and seasonal variations in IMF discontinuity occurrences have a significant correlation (0.4–0.5) with those of MIEs. We also find that MIEs tend to occur during intervals of low density, high‐speed solar wind streams, and/or radial IMF. No preferences for MIE occurrences are found for solar wind pressure jumps or IMF Bz southward turnings. From detailed minimum variance analysis of 36 IMF discontinuities with one‐to‐one correspondence to MIEs, ∼70% of the IMF discontinuities are found to be tangential discontinuities. The hot flow anomaly (HFA) mechanism can explain at most ∼50% of the MIEs; bursty reconnection and pressure pulses can explain the production of at most ∼30% and ∼20% of the MIEs, respectively. All of the observations and associations are consistent with HFAs or foreshock cavities being the main cause of MIEs.

Global‐scale simulation of foreshock structures at the quasi‐parallel bow shock

A two‐dimensional, global‐scale hybrid simulation is carried out to study the kinetic structure of the quasi‐parallel bow shock. In the simulation the bow shock forms by the interaction of the supersonic solar wind and the geomagnetic field. Strong temporal electromagnetic waves occur in the shock transition and foreshock regions due to backstreaming and reflected ions at the bow shock. This self‐consistent, global‐scale simulation shows the formation of diamagnetic cavities in the foreshock regions of the parallel and quasi‐parallel shocks, where ion beams interact with the incoming solar wind plasma. The cavities are crater‐like and saturate at a width ∼1–2 RE, with a low‐density and low‐magnetic field center bounded by a rim of high density and high magnetic field. The bulk flow speed decreases slightly in the center of the cavity, while the ion temperature often, but not always, increases. The craters convect downstream with the solar wind flow. When the interplanetary magnetic field (IMF) lies nearly parallel to the solar wind flow, the craters develop into elongated phase‐standing spatial structures along the field lines, both upstream and downstream, with alternate increases and decreases in the density and corresponding in‐phase variations in the magnetic field strength. In the general cases with an oblique IMF the foreshock cavities can convect toward the Earth as transient compressional structures and impinge on the magnetopause. The generation and structure of the diamagnetic cavities are compared with those of hot flow anomalies. The simulation results are also compared with satellite observations of the foreshock of quasi‐parallel shocks.

Observation of energy-time dispersed ion structures in the magnetosheath by CLUSTER: possible signatures of transient acceleration processes at shock

Abstract. We analyse energy-time dispersed ion signatures that have been observed by CLUSTER in the dayside magnetosheath. These events are characterized by sudden increases in the ion flux at energies larger than 10 keV. The high energy ions (30 keV) are first detected, with the transition to the low energy ions (5 keV) lasting about 100 s. These injections are often associated with transient plasma structures of a few minutes in duration, characterized by a hotter, less dense plasma and a diverted flow velocity, thus presenting similarities with "hot flow anomalies". They also involve modifications of the magnetic field direction, suggesting that the shock interacts with a solar wind discontinuity at the time of the event. The injections can originate from the magnetosphere or the shock region. Studying in detail a particular event, we discuss this last hypothesis. We show that the observed energy/time dispersion can be explained by combining a time-of-flight effect with a drift of the source of energetic particles along the shock. We propose that the acceleration results from a Fermi process linked to the interaction of the discontinuity with a quasi-perpendicular shock. This model explains the observed pitch-angle selection of the accelerated particles. The Fermi process acting on the beam of ions reflected from the shock appears to be sufficiently efficient to accelerate over short time scales (less than 30 s) particles at energies above 30 keV.Key words. Magnetospheric physics (solar-wind-magnetosphere interaction; magnetosheath) – Space plasma physics (shock waves)

Traveling convection vortices induced by solar wind tangential discontinuities

Two typical magnetic impulse events (MIEs) accompanied by traveling convection vortices (TCVs) are investigated. The analysis of their conjugate equivalent convection patterns is performed using magnetic field data obtained from high‐latitude ground magnetometer networks in the Northern and Southern Hemispheres. A three‐dimensional analysis of solar wind structures is also performed using solar wind data obtained from multiple International Solar‐Terrestrial Physics satellites. In the first event observed at ∼1310 UT on 22 May 1996, a westward moving TCV appeared simultaneously in the noon‐to‐dawn sector in the Northern and Southern Hemispheres. The solar wind source of this TCV is found to be a tangential discontinuity (TD), which causes a rapid northward turning of the interplanetary magnetic field (IMF) and abrupt dynamic pressure changes. In the second event observed at ∼1610 UT on 27 May 1998, an eastward moving TCV appeared in the noon sector in the Northern and Southern Hemispheres, with a timing delay of 2 to 3 min in the Southern Hemisphere. The solar wind source of this TCV is found again to be a TD, which causes a rapid IMF By negative turning and an abrupt enhancement of dynamic pressure. Analyses show that the TDs driving these events have their motional electric fields pointing toward the TDs and their normal vectors with large cone angles from the sunward direction. These TDs satisfy the conditions for the formation of a hot flow anomaly (HFA) at the bow shock. The sweeping motion across the magnetosphere of the intersection of the TD and the bow shock is found to be consistent with the observed TCV motion in each event. Magnetopause deformations due to HFAs can explain all the observed morphological features and the triggering process of these two MIEs. It is suggested, however, that bursty merging and/or pressure pulses would reinforce the processes produced by the HFAs, since the TDs are usually accompanied by both abrupt IMF changes and pressure enhancements. Consequently, it seems reasonable to conclude that the integrated processes of HFA, bursty magnetic field merging, and pressure pulses produce the evolution of these MIEs and TCVs.

The structure of hot flow anomalies in the magnetosheath

The interaction of the solar wind with the Earth's magnetosphere creates a population of backstreaming ions. When a tangential discontinuity contacts the bow shock, these ions can be focused to the discontinuity region and create so-called Hot Flow Anomalies (HFAs) - diamagnetic cavities filled with hot, tenuous, and deflected plasma population. These cavities are swept downstream and can be observed in the magnetosheath. We have analyzed INTERBALL-1 and MAGION-4 observations of magnetosheath HFAs with attention to their internal structure. We have found that HFAs often consist of two parts separated by an density enhancement. The particle behaviour can differ in these two parts. We demonstrate this peculiarity and discuss its possible origin.

Predictions of magnetosheath merging between IMF field lines of opposite polarity

Magnetohydrodynamic (MHD) simulations using the Integrated Space Weather Prediction Model (ISM) show merging in the magnetosheath between interplanetary magnetic field (IMF) field lines on opposite sides of a directional discontinuity. As the discontinuity passes the bow shock and traverses the magnetosheath, the magnetic field gradients on either side become steeper, and the geometry is distorted by the nonuniform flow speed in the magnetosheath. Associated current densities are intensified. In a location isolated from the magnetopause, we use the appearance of an X magnetic field configuration, associated dissipation electric fields, and increases in plasma velocity in the exhaust direction from the X as evidence for merging in the magnetosheath. We suggest that these signatures might be observable by Polar, as it now traverses the magnetosheath near the nose or by the Cluster spacecraft when the discontinuity is tilted away from the YZ plane. Merging signatures are seen in simulations with both 90° and 180° rotations of the field across the discontinuity. Magnetosheath merging creates a hole in the directional discontinuity, allowing the magnetosphere to penetrate through the structure as the discontinuity passes downstream. Magnetosheath merging has the potential to affect ionospheric convection pattern changes, open–closed boundaries, and magnetotail dynamics. A possible association with hot flow anomalies (HFA) is also indicated.

Wind observations of foreshock cavities: A case study

We present a case study of plasma, energetic particle, and magnetic field perturbations seen by Wind in the dawn foreshock on 19 April 1996. A comparison with the results of hybrid simulations confirms model predictions for a diamagnetic cavity flanked by regions of enhanced density and magnetic field strength but filled with a suprathermal ion population. In contrast to previously reported hot flow anomalies, flow velocities within the foreshock cavities are nearly identical to those in the ambient solar wind, ion temperatures do not rise greatly, thermal pressures are only slightly greater than those in the ambient solar wind, and the events do not lie centered upon interplanetary magnetic field (IMF) tangential discontinuities intersecting the bow shock. We attribute the cavities to the diamagnetic effects of ions Fermi accelerated within the foreshock. A review suggests that foreshock cavities are far more common than hot flow anomalies.

Global hybrid simulation of hot flow anomalies near the bow shock and in the magnetosheath

A two-dimensional global hybrid simulation is carried out to examine the generation of hot flow anomalies (HFAs) by interaction of the Earth's bow shock (BS), magnetosheath, and magnetosphere with an interplanetary tangential discontinuity (TD). By including the dayside magnetosphere in the simulation domain, this new study focuses on the formation and internal structure of strong HFAs, both near the bow shock and in the magnetosheath, and their effects on the dayside magnetopause. The simulation is performed for directional TDs, which carry a magnetic field orientation change and propagate to the BS from various directions. In the cases in which the magnetic field orientations across the TD result in a normal component of the upstream motional electric field pointing toward the TD, a strong HFA can be generated by coherent, gyrating reflected ion beams near the BS. In the center of the upstream part of the HFA, a low plasma density and a low magnetic field strength are present. The plasma is heated to a nearly isotropic temperature that is much greater than that in the downstream magnetosheath. The bulk flow speed is greatly reduced, and a significant sunward flow deflection is present in the HFA. The HFA bulges into the solar wind due to an enhancement of the total pressure in the hot cavity. As a result, the local BS and the magnetosheath may expand into the upstream solar wind by several Earth radii. A significant enhancement in the density and magnetic field strength flanks the bulge. The structure of the magnetosheath HFA, which encompasses the transmitted tangential discontinuity, is similar to that for the upstream HFA. Nevertheless, relative to the magnetosheath total pressure perpendicular to the HFA boundaries, a low pressure region is present in the magnetosheath HFA due to pressure balance with the upstream HFA. The magnetopause can expand sunward by several Earth radii as the HFA passes by. In the simulation, the size of the diamagnetic cavity increases with the rotation angle Δ Φ of the magnetic field through the TD, which corresponds to the magnitude of the normal electric field. The maximum strength of the HFA occurs when the angle of the propagation direction of the TD relative to the Sun–Earth line, γ, is nearly 80°. In addition, spatial structures with alternate temperature increases and decreases develop at the quasi-parallel shock. These structures are elongated along field lines, both upstream and downstream of the bow shock. Those with temperature increases and density, magnetic field, and the flow speed decreases appear like weak HFAs, but without strong flow deflections.

Nature of fluctuations on directional discontinuities inside a solar ejection: Wind and IMP 8 observations

A solar ejection passed the Wind spacecraft between December 23 and 26, 1996. On closer examination, we find a sequence of ejecta material, as identified by abnormally low proton temperatures, separated by plasmas with typical solar wind temperatures at 1 AU. Large and abrupt changes in field and plasma properties occurred near the separation boundaries of these regions. At the one boundary we examine here, a series of directional discontinuities was observed. We argue that Alfvénic fluctuations in the immediate vicinity of these discontinuities distort minimum variance normals, introducing uncertainty into the identification of the discontinuities as either rotational or tangential. Carrying out a series of tests on plasma and field data including minimum variance, velocity and magnetic field correlations, and jump conditions, we conclude that the discontinuities are tangential. Furthermore, we find waves superposed on these tangential discontinuities (TDs). The presence of discontinuities allows the existence of both surface waves and ducted body waves. Both probably form in the solar atmosphere where many transverse nonuniformities exist and where theoretically they have been expected. We add to prior speculation that waves on discontinuities may in fact be a common occurrence. In the solar wind, these waves can attain large amplitudes and low frequencies. We argue that such waves can generate dynamical changes at TDs through advection or forced reconnection. The dynamics might so extensively alter the internal structure that the discontinuity would no longer be identified as tangential. Such processes could help explain why the occurrence frequency of TDs observed throughout the solar wind falls off with increasing heliocentric distance. The presence of waves may also alter the nature of the interactions of TDs with the Earth's bow shock in so‐called hot flow anomalies.

Hot diamagnetic cavities upstream of the Martian bow shock

We present Mars Global Surveyor (MGS) observations of hot diamagnetic cavities upstream of the Martian bow shock. The events are characterized by a region of turbulent magnetic field, high electron temperature, and densities at or below the ambient solar wind density. The region of high temperature is flanked by layers of high magnetic field and electron density and is accompanied by a rotation in the interplanetary magnetic field (IMF). We suggest that the events are generated by interplanetary discontinuities interacting with the Martian bow shock, i.e., the events are the Martian counterpart to the hot flow anomalies observed upstream of the Earth's bow shock.

Magnetopause motion driven by interplanetary magnetic field variations

We use previously reported observations of hot flow anomalies (HFAs) and foreshock cavities to predict the characteristics of corresponding features in the dayside magnetosheath, at the magnetopause, and in the outer dayside magnetosphere. We compare these predictions with Interball 1, Magion 4, and GOES 8/GOES 9 observations of magneto‐pause motion on the dusk flank of the magnetosphere from 1800 UT on January 17 to 0200 UT on January 18, 1996. As the model predicts, strong (factor of 2 or more) density enhancements bound regions of depressed magnetosheath densities and/or outward magnetopause displacements. During the most prominent event, the geosynchronous spacecraft observe an interval of depressed magnetospheric magnetic field strength bounded by two enhancements. Simultaneous Wind observations indicate that the intervals of depressed magnetosheath densities and outward magnetopause displacements correspond to periods in which the east/west (By) component of the interplanetary magnetic field (IMF) decreases to values near zero rather than to variations in the solar wind dynamic pressure, the north/south component of the IMF, or the IMF cone angle.

Conditions for the formation of hot flow anomalies at Earth's bow shock

Hot flow anomalies (HFAs) result from the interaction of an interplanetary current sheet with Earth's bow shock and were discovered over a decade and a half ago. The deflected flow and hot interior of an HFA are consequences of ions reflected at the bow shock being channeled along the current sheet. Previous studies have shown that this requires a solar wind motional electric field pointing toward the current sheet on at least one side and that the current sheet must be a tangential discontinuity. Recent reports of a rapid displacement of the magnetopause by 5 Re as the result of an HFA have led us to explore the interplanetary conditions surrounding all reported HFAs. The kinetic aspects of HFA formation suggest that current sheets should pass relatively slowly along the bow shock; that is, their normals should have large cone angles. This hypothesis is confirmed. Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most HFAs have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar‐terrestrial dynamics.

Ground magnetic response to sudden changes in the interplanetary magnetic field orientation

Recent observations have shown that a discontinuity in the interplanetary magnetic field (IMF) can produce a discernible ground magnetic signature at cusp latitudes, whose characteristics closely resemble those of ionospheric traveling convection vortices. It has been suggested that a hot flow anomaly (HFA), produced by the interaction of certain tangential discontinuities with the bow shock, is the mechanism through which an IMF discontinuity can produce a ground magnetic response. Using solar wind plasma and IMF data from the IMP 8 satellite from 1992–1995, we have identified and categorized intervals during which IMF discontinuities are observed. Ground magnetometer observations from the Greenland coastal chains and the MAGIC array have been used to characterize any resulting ground response. The occurrence of ground signatures appears to be weakly related to the conditions necessary to produce HFAs.

Comprehensive study of the magnetospheric response to a hot flow anomaly

We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball‐1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (∼7 min) move outward some 5 RE beyond its nominal position and engulf Interball‐1. A timing study employing nearby Interball‐1 and Magion‐4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES‐8 geosynchronous observations near local dawn.Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near‐simultaneous perturbations in high‐latitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind‐magnetosphere interaction.

Gross deformation of the dayside magnetopause

On July 24, 1996, the entire magnetosheath moved past the Interball‐1 spacecraft twice within 7 min. Such rapid transits through the magnetosheath suggest the passage of an antisunward‐moving boundary wave with an amplitude in excess of 5 RE. A wave of this amplitude requires an order of magnitude decrease in the pressure confining the magnetosphere. Such decreases have been reported only in conjunction with hot flow anomalies, features accompanying interplanetary magnetic field discontinuities, bounded by narrow shocks, and identified on the basis of heated deflected flows transverse to the Earth‐Sun line. We identify all these features just outside the Interball‐1 magnetopause crossings on July 24, 1996.