Traveling Convection Vortices Research Articles

A statistical study of traveling convection vortices using the Magnetometer Array for Cusp and Cleft Studies

The Magnetometer Array for Cusp and Cleft Studies (MACCS) is primarily a longitudinal magnetometer array located in the eastern Canadian arctic at cusp latitudes. We present a statistical study of traveling convection vortices (TCVs) as observed by MACCS from the first year of operations, August 1992 to July 1993. For this period, data are routinely available from three stations, extending more than 2 hours in magnetic local time (MLT) and located roughly on a line of constant geomagnetic latitude at 75°. With the selection criteria used, we find an average of 38 TCV events per month. We characterize the events as “isolated” and “nonisolated”. The isolated events have the typical, accepted signature of a TCV while the nonisolated ones are events that satisfy the selection criteria but occur among relatively strong magnetic activity. The diurnal distribution of these events shows a prenoon‐afternoon asymmetry, but the distribution has a single peak at 10 MLT and then falls off smoothly in the afternoon hours. It does not show the previously reported minimum of event occurrence around magnetic local noon except for isolated events occurring during higher Kp values. The most likely explanation for this discrepancy is the wide longitudinal extent of MACCS. The majority of the events have propagation velocities between 3 and 11 km/s, which can be a factor of 2 larger than the events reported in previous studies. The majority of the events move away from noon, as is expected, with a small fraction moving toward noon. Most (80%) of the studied events exhibit all the characteristics of a typical TCV, while the remaining 20% show some form of irregularity. We find four different types of irregularities. The isolated and nonisolated events have many similar morphological and propagation characteristics. However, quantitative comparison of the statistical distributions of the two types of TCVs indicates that they belong to different populations. We also find that nonisolated TCVs may occur for somewhat higher Kp values. We therefore suggest that the two classes of TCVs have different source mechanisms.

Conjugate observations of traveling convection vortices: The field‐aligned current system

Analysis of a single traveling convection vortex (TCV) event that combines magnetometer, auroral imager, and riometer observations from conjugate hemispheres provides a complete view of the field‐aligned current (FAC) system associated with the TCV. Conjugate observations clearly show that the TCV is a pair of FACs that flow alternatively out of and then into both ionospheres. Imaging observations at 427.8 nm indicate that the upward FAC region is associated with a narrow arc 50–100 km wide and at least 600 km long. Mapping the imaging observations made in the Southern Hemisphere to the Northern Hemisphere shows excellent collocation of the precipitation with the flow vortex associated with the upward FAC as inferred from a large two‐dimensional network of magnetometers. Also consistent with the flows inferred from the magnetometers, the arc is found to be skewed with respect to the direction of motion of the transient. Low‐altitude spacecraft passes near the time of the event suggest that the FAC is coincident with, and possibly confined to, a relatively narrow region (less than 3° in latitude) in which 1–10 keV electrons characteristic of the central plasma sheet are present.

Signatures of traveling convection vortices in ground magnetograms under the equatorial electrojet

Traveling convection vortices (TCVs) are transient events in high‐latitude ground magnetograms that can be interpreted as evidence for localized vortical flows in the high‐latitude ionosphere. Their centers travel eastward or westward at geomagnetic latitudes near 72° to 74°. TCVs have been attributed to various transient interactions at the magnetopause but also to processes occurring deeper within the magnetosphere. Each interaction mechanism should launch shear Alfvén waves that produce high‐latitude ground signatures but also compressional waves that should produce signatures at lower latitudes. Zesta et al. [1999], Moretto et al. [1997], Ridley et al. [1998], and Lühr et al. [1998] report detailed analyses for several high‐latitude TCVs. In this paper, we present evidence for corresponding signatures at Belem, São Luis, and Teresina and other stations under or nearby the equatorial electrojet. The equatorial signatures are most pronounced when the high‐latitude signatures are (1) longitudinally extended, (2) isolated, and (3) attain greatest amplitudes.

Magnetospheric signature of an ionospheric traveling convection vortex event

We analyze Geotail observations of magnetic field and flow oscillations in the dayside magnetosphere that occurred in direct association with an event of traveling convection vortices observed in the northern high‐latitude ionosphere. Geotail was positioned on a magnetic flux tube that mapped very close to the centers of the driving field‐aligned currents in the ionosphere. These are the first in situ observations from the source region of such events. The flow signature is seen clearly in the cold, dense plasma population, which the satellite encounters during the event, in addition to a hot magnetospheric (plasma sheet‐like) population. This report will present the observations and briefly discuss their implications for our understanding of the traveling convection vortex (TCV) signature. We interpret the event in terms of a boundary wave at the magnetopause. Our analysis suggests that strong gradients in the magnetic field strength or plasma density in the inner magnetosphere should play an important role in this process.

Polar UVI observations of dayside auroral transient events

We analyze Polar Ultraviolet Imager (UVI) observations of auroral transient events (ATEs) in the dayside Northern Hemisphere. During 5 winter months in 1996 and 1997, we found 31 prenoon ATEs but only 13 afternoon events. Prenoon and afternoon event characteristics differ. Prenoon ATEs generally appear as bright spots of auroral luminosity in the area from 0800 to 1000 magnetic local time (MLT) and 74.5° and 76.5° corrected geomagnetic latitude (CGL). Bright aurorae then quickly expand westward and poleward, accompanied by high‐latitude magnetic impulsive events (MIE) and traveling convection vortices (TCV). Afternoon ATEs usually appear as a sudden intensification of aurorae in the area from 1400 to 1600 MLT and 75.5° to 78.5° CGL. Within 15‐20 min the bright band of luminosity extends eastward to reach 2000–2100 MLT at 70°–72° CGL. Although midlatitude and low‐latitude ground magnetograms in the evening sector record increases in the horizontal component of the magnetic field, no corresponding features occur at stations in the morning sector. Afternoon ATEs correspond to abrupt changes in the interplanetary magnetic field (IMF) orientation, but not to significant variations of the solar wind dynamic pressure, indicating that the auroral transient events occur as part of the magnetospheric response to abrupt changes in the foreshock geometry.

Solar wind effects on ionospheric convection: a review

The solar wind, magnetosphere, and ionosphere are intrinsically coupled through magnetic field lines. The electrodynamic state of the high-latitude ionosphere is controlled by several geophysical processes, such as the location and rate of magnetic reconnection at the magnetopause and in the magnetotail, and the energisation and precipitation of solar wind and magnetospheric plasmas. Amongst the most observed ionospheric manifestation of solar wind/magnetospheric processes are the convection bursts associated with the so-called flux transfer events (FTEs), magnetic impulse events (MIEs), and travelling convection vortices (TCVs). Furthermore, the large-scale ionospheric convection configuration has also demonstrated a strong correspondence to variations in the interplanetary medium and substorm activity. This report briefly discusses the progress made over the past decade in studies of these transient convection phenomena and outlines some unsettled questions as well as future research directions.

Magnetic impulse event: A detailed case study of extended ground and space observations

Analysis of conjugate data from extended magnetometer networks in northern and southern high latitudes is used to elucidate the initiation and the evolution of a magnetic impulse event (MIE) on June 6, 1997. In addition, data from all‐sky imagers, imaging riometers, and Super Dual Auroral Radar Network radars in Antarctica are investigated to confirm the energy content, motion, and electrical current structure of the MIE. The MIE was accompanied by traveling convection vortices (TCVs) that began at ∼10 MLT and moved eastward (toward dusk) and slightly equatorward at 1–3 km/s across the noon meridian with north‐south conjugacy. The MIE had upward field‐aligned currents with soft electron precipitation that was located near the trailing edge of the Hall current loop. During the MIE interval the interplanetary magnetic field (IMF) was directed strongly outward from the Sun (Bx = −5 nT), with a slightly positive (1–2 nT) Bz, and a nearly zero By. Since abrupt solar wind pressure changes are unlikely under this IMF orientation (and none was, in fact, observed), classical mechanisms for MIE generation, such as a pressure pulse or dayside reconnection, are excluded. It is speculated that an abrupt IMF cone angle change from 60° to 20°, ∼30 min prior to the MIE onset, may have been an indirect trigger of this event via the interaction between the solar wind and the bow shock.

Testing models for traveling convection vortices: Two case studies

We use geosynchronous magnetic field observations to discriminate between two models for traveling convection vortices (TCVs) generated by pressure variations applied to the magnetosphere. According to the model of Glassmeier [1992], an azimuthal gradient in the pressure generates a single field‐aligned current into the high‐latitude ionosphere and a single TCV. According to the model of Southwood and Kivelson [1990], an azimuthal gradient in the pressure generates a pair of oppositely directed currents and TCVs. We reexamine two well‐documented examples of TCVs and show that their signatures correspond better to the predictions of the Southwood/Kivelson model than to the predictions of the Glassmeier model.

The November 9, 1993, traveling convection vortex event: A case study

On November 9, 1993, at around 1715 UT, a strong and well‐structured traveling convection vortex (TCV) event occurred and was observed by the Magnetometer Array for Cusp and Cleft Studies (MACCS) and the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer chains. The Greenland chain, which is located farther to the east, observed only a very weak signature for the same event. We studied the propagation characteristics of the two‐dimensional vortical TCV current patterns. The TCV event is a series of four vortical currents propagating westward. We found that individual vortices propagate with different speeds. The vortices are created in the early postnoon region. They accelerate as they strengthen, and some decelerate, weaken, and disappear within the 6 hours of magnetic local time of the field of view of the ground magnetometers. The strongest, main vortex of the event accelerates until it moves out of the field of view and, more than likely, reaches well into the nightside. We studied the correlated solar wind and IMF signatures as observed by the IMP 8 spacecraft, sitting in the far dawnside and outside the bow shock. We found that the transient currents in the ionosphere are the result of sharp, short‐lived pressure pulses hitting the magnetopause during times of quiet and northward IMF that is primarily radial. We find that the pressure pulses are more than likely created just upstream of the bow shock by the interaction of the quasi‐parallel shock with IMF orientation changes and are not intrinsic features of the upstream solar wind. We also analyze the transient signatures in the inner magnetosphere by studying the magnetic field data in the GOES 6 and 7 satellites. We suggest that a series of five successive compression and depression peaks in the GOES magnetic field data are well correlated with the set of solar wind pressure pulses. We observe a propagation velocity of the transient event from the GOES 6 spacecraft to the GOES 7 spacecraft that agrees well with the propagation velocities that we calculate from the ground magnetometer stations.

Response of the ionosphere to a density pulse in the solar wind: Simulation of traveling convection vortices

We show the response of the Earth's magnetosphere and ionosphere to a density pulse in the solar wind using a global 3D MHD simulation model. Flow vortices are generated in the ionosphere and they exhibit many properties similar to those observed during impulsive traveling convection vortices. Two oppositely rotating flow vortices are formed at about 70° magnetic latitude near noon. They separate and move down the morning and evening flanks, greatly weakening as they pass the terminator. Meanwhile a second pair of flow vortices appears at noon at a slightly higher latitude and with opposite flow directions than the first pair. The second pair follows the first and also fades as it reaches the nightside. The results are interpreted as a hydromagnetic wave propagating in the inhomogeneous magnetosphere plasma.

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.

What are traveling convection vortices?

Traveling convection vortices (TCV) are studied using a theoretical ideal MHD model for the equilibrium and perturbed plasma. In this first approach, we do not consider the viscosity and the flow of the plasma column in the equilibrium. The linearized equations are solved using normal mode analysis. The solutions show an m = 1 kink instability which is in good agreement with the experimental data. It is calculated numerically a growth rate of 28 minutes to be compared with experimental data, indicating a quite good result with our model. Clearly, the results indicate the behaviour of the TCV as a kind of kink instability as supported by the agreement between theoretical and experimental values for the growth rate. Also the filamentation currents calculated through the theoretical model agree quite well with the experimental observations by satellites.

Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets

Abstract. We report important results of the first campaign specially designed to observe the formation and the initial convection of polar cap patches. The principal instrumentation used in the experiments comprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. The experiment was conducted on February 18, 1996 and was complemented with additional sensors such as the Greenland chain of magnetometers and the WIND and IMP-8 satellites. Two different types of events were seen on this day, and in both events the Sondrestrom radar registered the formation and evolution of large-scale density structures. The first event consisted of the passage of traveling convection vortices (TCV). The other event occurred in association with the development of large plasma jets (LPJ) embedded in the sunward convection part of the dusk cell. TCVs were measured, principally, with the magnetometers located in Greenland, but were also confirmed by the line-of-sight velocities from the Sondrestrom and SuperDARN radars. We found that when the magnetic perturbations associated with the TCVs were larger than 100 nT, then a section of the high-latitude plasma density was eroded by a factor of 2. We suggest that the number density reduction was caused by an enhancement in the O+ recombination due to an elevated Ti, which was produced by the much higher frictional heating inside the vortex. The large plasma jets had a considerable (>1000 km) longitudinal extension and were 200-300 km in width. They were seen principally with the Sondrestrom, and SuperDARN radars. Enhanced ion temperature (Ti) was also observed by the Sondrestrom and EISCAT radars. These channels of high Ti were exactly collocated with the LPJs and some of them with regions of eroded plasma number density. We suggest that the LPJs bring less dense plasma from later local times. However, the recent time history of the plasma flow is important to define the depth of the density depletion. Systematic changes in the latitudinal location and in the intensity of the LPJs were observed in the 2 min time resolution data of the SuperDARN radars. The effect of the abrupt changes in the LPJs location is to create regions containing dayside plasma almost detached from the rest of the oval density. One of these density features was seen by the Sondrestrom radar at 1542 UT. The data presented here suggest that two plasma structuring mechanisms (TCVs and LPJs) can act tens of minutes apart to produce higher levels of density structures in the near noon F-region ionosphere.Key words. Ionosphere (ionospheric irregularities) · Magnetospheric physics (electric fields; polar cap phenomena)

Effects of magnetospheric precipitation and ionospheric conductivity on the ground magnetic signatures of traveling convection vortices

By using an improved TCV model (Zhu et al., 1997), a quantitative study of the effects of magnetospheric precipitation and ionospheric background conductivity on the ground magnetic signatures of traveling convection vortices (TCVs) has been conducted. In this study the localized conductivity enhancement associated with the TCVs is present and the ratio of the Hall and Pedersen conductances vary both spatially and temporally according to the hardness of the TCV precipitation. It is found that a strong conductivity enhancement associated with hard TCV precipitation can significantly distort the TCV current closure in the ionosphere and lead to ground magnetic disturbance patterns with strong asymmetry in E–W direction. The asymmetry of the ground magnetic patterns is characterized by a stronger magnetic disturbance on the side of the upward field‐aligned currents (clockwise convection cell) and a possible rotation of the whole magnetic patterns. Specifically, the modeling results predict that when the characteristic energy of the TCV precipitation is below 500 eV, the asymmetry of the ground magnetic patterns is minimal (less than 1%) and may not be detectable. When the characteristic energy of the precipitation is about 7 keV, the asymmetry of the magnetic patterns can be well above 30%. It is also found that a low ionospheric background conductivity favors the appearance of strong asymmetry in the ground magnetic patterns of the TCVs, while a high ionospheric background conductivity favors the appearance of strong ground magnetic disturbances but with less asymmetry. We concluded that the most favorable condition for the appearance of strong asymmetry in the TCV ground magnetic signatures is the condition of winter, solar minimum, and hard precipitation.

A dayside ionospheric absorption perturbation in response to a large deformation of the magnetopause

A large deformation of the dayside magnetopause was observed by Interball‐1 from 1132 to 1139 UT on July 24, 1996. The spacecraft, near 0800 LT at the time, transited the magnetosheath twice within 7 minutes. In response to these boundary perturbations, the antarctic stations South Pole and AGO‐P3, located at about the same local time as Interball‐1, observed impulsive magnetic variations. These variations were similar to previously reported magnetic variations observed in the northern hemisphere conjugate region during this event and may be related to discrete field‐aligned currents linking the ionosphere to the perturbed outer magnetosphere. Westward propagation of the magnetic signatures in both ionospheric regions is consistent with a travelling convection vortex event; however, the speed is significantly higher in the north (∼10 km/s) than in the south (∼3 km/s). A localized intensification of energetic electron precipitation (427.8 nm auroral emission and riometer absorption) was observed at South Pole station, but not at AGO‐P3 or at the nominally conjugate locations in Greenland and Canada. The complex, Z‐component variation of the magnetic pulse accompanying the particle precipitation at South Pole may be evidence of a localized ionospheric conductivity enhancement, as proposed in a recent model study of travelling convection vortices.

Traveling convection vortices as seen by the SuperDARN HF radars

Two impulsive traveling convection vortex (TCV) events observed simultaneously by ground based magnetometers and the SuperDARN HF radars in the prenoon sector were studied. In both cases, disturbances traveled westward at speeds of 4–6 km/s. Convection patterns derived from magnetometer measurements and radar observations were overall in reasonable agreement; observed differences at some points might be caused by both the nonuniform ionospheric conductivity distribution and difference in the integration time of the radar and magnetometer data. For one event, the convection patterns obtained from magnetometer data and SuperDARN radar measurements were relatively simple; they can be interpreted as a result of the westward motion of a convection vortex system associated with a pair of field‐aligned currents separated in azimuthal direction. This TCV event was associated with relatively low Pc5 pulsation activity, contrary to the second TCV event that was accompannied by a train of Pc5 magnetic pulsations of large amplitude. Convection patterns for the second event were complicated. A simple scenario for the interpretation of the generation of TCVs and Pc5 pulsations is suggested. A sudden impulse in the solar wind dynamic pressure produces disturbances on several boundaries of magnetospheric plasma: on the magnetopause, the LLBL inner edge, and the plasma sheet inner edge. These boundaries are elastic so that surface waves can propagate along them. The high‐latitude wave is responsible mainly for TCVs, whereas the low‐latitude waves may be responsible for excitation of Pc5 field line resonance pulsations. The scenario explains important features of both TCV events and Pc5 pulsations: both phenomena appear simultaneously and show westward (eastward) propagation, but the TCVs are observed at latitudes close to the LLBL inner edge, whereas the Pc5 pulsations occur at lower latitudes, close to the inner boundary of the plasma sheet.

Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets

We report important results of the first campaign specially designed to observe the formation and the initial convection of polar cap patches. The principal instrumentation used in the experiments comprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. The experiment was conducted on February 18, 1996 and was complemented with additional sensors such as the Greenland chain of magnetometers and the WIND and IMP-8 satellites. Two different types of events were seen on this day, and in both events the Sondrestrom radar registered the formation and evolution of large-scale density structures. The first event consisted of the passage of traveling convection vortices (TCV). The other event occurred in association with the development of large plasma jets (LPJ) embedded in the sunward convection part of the dusk cell. TCVs were measured, principally, with the magnetometers located in Greenland, but were also confirmed by the line-of-sight velocities from the Sondrestrom and SuperDARN radars. We found that when the magnetic perturbations associated with the TCVs were larger than 100 nT, then a section of the high-latitude plasma density was eroded by a factor of 2. We suggest that the number density reduction was caused by an enhancement in the O+ recombination due to an elevated Ti, which was produced by the much higher frictional heating inside the vortex. The large plasma jets had a considerable (>1000 km) longitudinal extension and were 200-300 km in width. They were seen principally with the Sondrestrom, and SuperDARN radars. Enhanced ion temperature (Ti) was also observed by the Sondrestrom and EISCAT radars. These channels of high Ti were exactly collocated with the LPJs and some of them with regions of eroded plasma number density. We suggest that the LPJs bring less dense plasma from later local times. However, the recent time history of the plasma flow is important to define the depth of the density depletion. Systematic changes in the latitudinal location and in the intensity of the LPJs were observed in the 2 min time resolution data of the SuperDARN radars. The effect of the abrupt changes in the LPJs location is to create regions containing dayside plasma almost detached from the rest of the oval density. One of these density features was seen by the Sondrestrom radar at 1542 UT. The data presented here suggest that two plasma structuring mechanisms (TCVs and LPJs) can act tens of minutes apart to produce higher levels of density structures in the near noon F-region ionosphere.Key words. Ionosphere (ionospheric irregularities) · Magnetospheric physics (electric fields; polar cap phenomena)

Global analysis of three traveling vortex events during the November 1993 storm using the assimilative mapping of ionospheric electrodynamics technique

We have studied three traveling vortex events during the November 3–11, 1993, storm period using the assimilative mapping of ionospheric electrodynamics technique to help interpret the ground magnetic response from a global set of stations. When the AMIE potential patterns are high‐pass filtered, clear vortex structures are observed. The AMIE output compares reasonably well with the ground magnetic data, although AMIE has a tendency to smooth out the structures. Single vortices are observed to form near 1000 MLT and propagate to 1400 MLT, over the Greenland magnetometers and across noon. The velocity of the vortices is approximately 4–8 km/s (0.2° – 0.3° s−1 between 70° and 75°) eastward along a line of invariant latitude. The number of vortices, as well as the motion of the vortices, differs from the classical picture of a traveling convection vortex. We therefore suggest that these vortex events might have a different generation mechanism or occur under a different state of the magnetosphere, and thus that they should be considered a separate class of events. The AMIE technique offers the possibility to relate the vortices to the large‐scale convection patterns. We note how the vortices can be interpreted as small scale, rapid changes in the large‐scale convection pattern, although these changes show many differences to previous studies of changing large‐scale convection patterns. We speculate that this particular class of traveling vortex event may be generated by a perturbation in the separatric on the dayside magnetopause, caused by an enhancement or depreciation of the magnetic merging.

Comment on “On the source region of traveling convection vortices” by A. G. Yahnin et al.

The source regions of traveling convection vortices (TCV) observed in the dayside magnetosphere outside the near noon sector have been mapped to magnetospheric locations within the outer radiation belt, but not to the Central Plasma Sheet (CPS) as claimed by Yahnin and Moretta [1996] and Yahnin et al. [1997]. Their suggested connection between TCV and the CPS is based on the unjustified application of terminology traditionally used for the auroral precipitation structure in the nightside magnetospheric sector [Winningham et al. 1975] to the late morning‐noon sector. This is a case where an important finding on the location of TCV with respect to natural magnetospheric boundaries has been presented using misleading terminology, which leads to an incorrect physical interpretation of the result. Hence this Comment emphasizes the need for a critical reexamination and modification of the “traditional” terminology concerning the plasma domain structure in the magnetosphere.

Multi‐instrument analysis of the ionospheric signatures of a hot flow anomaly occurring on July 24, 1996

We present the analysis of a coordinated set of observations from the POLAR ultraviolet imager (UVI), ground magnetometers, incoherent scatter radar, solar wind monitors, and the DMSP satellite, focused on a traveling convection vortex (TCV) event on July 24, 1996. Starting at approximately 1036 UT, ground magnetometers in Greenland and eastern Canada observe pulsations consistent with the passing overhead of a series of TCV field‐aligned current pairs. Azimuthal scans by the Sondrestrom incoherent scatter radar located near Kangerlussuaq (formerly Sondrestrom), Greenland, at this time show strong modulation in the strength and direction of ionospheric plasma flow. The magnetometer pulsations grow in magnitude over the next hour, peaking in intensity at 1137 UT. Images from the UVI instrument show a localized intensification of auroral emissions over central and western Greenland at 1139 UT. Subsequent images show the intensification grow in strength and propagate westward (tailward) until approximately 1158 UT, at which time the intensification fades. These observations are consistent with the westward passage of four pairs of TCVs over central Greenland. The intensification of auroral emissions at 1139 UT is associated with the leading vortex of the fourth TCV pair, thought to be the result of an upward field‐aligned current. The modulated flow observed by the radar is the result of the strong electric fields associated with the field‐aligned current systems responsible for the impulsive TCV as they pass through the field of view of the radar. Measurements taken in the solar wind by the Wind spacecraft suggest that a pressure change triggers the onset of TCV activity. A subsequent sudden change in the orientation of the interplanetary magnetic field produces a hot flow anomaly which forms at the bow shock. We believe that the interaction of the hot flow anomaly with the magnetopause intensified the fourth TCV pair and produced the associated auroral brightening. DMSP particle data indicate that the TCVs occur on field lines which map to the boundary plasma sheet‐low latitude boundary layer interface. The ground observations associated with the hot flow anomaly are the first of their kind and provide a mechanism to tie an interplanetary magnetic field orientation change into the existing theory that TCVs result from a deformation of the magnetopause.