Northern Cusp Research Articles

TRICE‐2/SuperDARN Observations and Comparison With the Associated MMS Magnetopause Crossing

AbstractTwo sounding rockets, designated TRICE‐2, were launched on 8 December 2018 into the northern cusp region. The two rockets were designated the high‐ and low‐flyers, respectively, and launched 2 min apart to investigate cusp structures, specifically their spatial or temporal nature. 2 hr prior to the cusp encounter by the TRICE‐2 rockets, the MMS satellites, located in the magnetopause boundary layer, observed switching ion beams under very similar IMF conditions as later observed by TRICE‐2. The observed ion beam switch in the boundary layer defined the location of the primary dayside X‐line. Both, TRICE‐2 and MMS, also observed the signatures of multiple X‐lines at the magnetopause, overlapping ion‐energy dispersions in the cusp and counterstreaming ion beams in the magnetopause boundary layer, respectively. In addition to the TRICE‐2 cusp observations, ionospheric convection patterns from the SuperDARN radar are used to explain the vastly different cusp ion signatures observed by the TRICE‐2 rockets. While the high‐flyer rocket progressed north through the center of the cusp, the low‐flyer rocket drifted off to the east and crossed into the dusk convection cell, traveling perpendicular to the ionospheric convection direction before reaching the poleward oriented section of the convection cell also observed by the high‐flyer counterpart.

Statistical analysis of overlapping double ion energy dispersion events in the northern cusp

This article presents a statistical analysis of overlapping double ion-energy dispersion events in the northern cusp (“double dispersion”). Double dispersion in either cusp is a phenomenon associated with multiple reconnections occurring on the dayside magnetosphere as a result of its constant interaction with the variable solar wind. Using observations from a low Earth orbiting (LEO) Defense Meteorological Satellite Program (DMSP) satellite, we analyze 138 dayside events selected by the automatic algorithm extended from our previous work. We conducted a correlation study between the number of detected double dispersion events and 1) the month of the year to analyze the seasonal response of the cusp, and 2) solar wind interplanetary magnetic field (IMF) components and clock/cone angles to investigate its relationship with magnetic reconnection. We found that dispersion events occur more frequently during the northern summer months (i.e., when the dipole is tilted Sunward) and when the By component of IMF is positive. In addition, we provide a machine-readable list of the events and the code used to automatically detect the events.

Flight demonstration of a miniature atomic scalar magnetometer based on a microfabricated rubidium vapor cell.

We have developed an atomic magnetometer based on the rubidium isotope 87Rb and a microfabricated silicon/glass vapor cell for the purpose of qualifying the instrument for space flight during a ride-along opportunity on a sounding rocket. The instrument consists of two scalar magnetic field sensors mounted at 45° angle to avoid measurement dead zones, and the electronics consist of a low-voltage power supply, an analog interface, and a digital controller. The instrument was launched into the Earth's northern cusp from Andøya, Norway on December 8, 2018 on the low-flying rocket of the dual-rocket Twin Rockets to Investigate Cusp Electrodynamics 2 mission. The magnetometer was operated without interruption during the science phase of the mission, and the acquired data were compared favorably with those from the science magnetometer and the model of the International Geophysical Reference Field to within an approximate fixed offset of about 550 nT. Residuals with respect to these data sources are plausibly attributed to offsets resulting from rocket contamination fields and electronic phase shifts. These offsets can be readily mitigated and/or calibrated for a future flight experiment so that the demonstration of this absolute-measuring magnetometer was entirely successful from the perspective of increasing the technological readiness for space flight.

Hybrid Simulations of the Cusp and Dayside Magnetosheath Dynamics Under Quasi-Radial Interplanetary Magnetic Fields.

Under quasi-radial interplanetary magnetic fields (IMF), foreshock turbulence can have an impact on the magnetosheath and cusps depending on the location of the quasi-parallel shock. We perform three-dimensional simulations of Earth's dayside magnetosphere using the hybrid code HYPERS, and compare northward and southward quasi-radial IMF configurations. We study the magnetic field configuration, fluctuations in the magnetosheath and the plasma in the regions around the northern cusp. Under northward IMF with Earthward B x , there is a time-varying plasma depletion layer immediately outside the northern cusp. In the southward IMF case, the impact of foreshock turbulence and high-speed jets, together with magnetopause reconnection, can lead to strong density enhancements in the cusp.

Magnetotail Configuration Under Northward IMF Conditions

AbstractDue to the sparsity of space probes, it is still not clear on how the magnetic structure of the magnetotail looks like and how it evolves when the interplanetary magnetic field (IMF) directs northward. This simulation study uses two different global magnetosphere magnetohydrodynamics (MHD) models to simulate two northward IMF events and study the evolution of the magnetotail. Both models show that the magnetotail may form a structure that is composed of a dawnside tail lobe and a duskside tail lobe, under northward IMF conditions with significant By, instead of a northern tail lobe and a southern tail lobe under southward IMF conditions. In this magnetic configuration, a tail lobe extends a domain from northern (southern) cusp to southern (northern) IMF. The larger the magnitude of IMF clock angle, the longer and wider the magnetotail. Such magnetic configuration suggests that magnetotail reconnection is possible to occur when the dawnside tail lobe contacts with the duskside tail lobe and thus a substorm is also possible to occur under northward IMF conditions with significant By.

The Shape of Mercury's Magnetopause: The Picture From MESSENGER Magnetometer Observations and Future Prospects for BepiColombo

AbstractThe MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft orbited Mercury between March 2011 and April 2015, typically passing through the magnetosphere, magnetosheath, and interplanetary magnetic field, on each orbit. Using data from the Magnetometer, we identify magnetopause crossings for the complete orbital mission and model the average boundary shape. We find that Mercury's average magnetopause is well modeled by both an axisymmetric shape and a three‐dimensional shape containing indentations in the cusp regions and a magnetotail that is wider in the north‐south versus east‐west direction. Examination of MESSENGER data and the use of simulated crossings show that coverage is very limited in the cusp region. Parameters describing the indentation are poorly constrained, and model fits may be biased due to the combination of orbital geometry and magnetospheric dynamics. The distribution of MESSENGER data inside the magnetopause, together with minimum variance analysis of a representative subset of magnetopause crossings, hints at the presence of a cusp indentation, though likely shallower than that estimated from the observed crossing positions. The BepiColombo spacecraft will arrive at Mercury in 2025. We simulate expected data coverage and magnetopause crossings using both axisymmetric and three‐dimensional magnetopause models. BepiColombo will improve our understanding of the magnetopause shape by providing data over the southern hemisphere and regions of the northern cusp indentation not covered by MESSENGER. There is also the potential for simultaneous measurements of the interplanetary magnetic field and magnetopause position.

Statistical study of ICME effects on Mercury's magnetospheric boundaries and northern cusp region from MESSENGER

AbstractThis paper presents a systematic investigation of the large‐scale processes in Mercury's magnetosphere during interplanetary coronal mass ejections (ICMEs) using observations from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. We study the motion of the bow shock and magnetopause boundaries, quantify the magnetospheric compression, and characterize the size, extent, and plasma pressure of the northern cusp region and the plasma precipitation to the surface. During ICMEs, the magnetopause was substantially compressed, as the subsolar standoff distance from the center of the planet was reduced by ∼15% compared with the value during nominal solar wind conditions, and the magnetopause reached the surface of the planet ∼30% of the time. On the other hand, the bow shock under ICME conditions was located farther from the planet than for nominal solar wind conditions. The cusp was observed to extend ∼10° farther equatorward and 2 h wider in local time. In addition, the average plasma pressure in the cusp was more than double that determined under nominal conditions. For the most extreme cases, the particle precipitation to the surface was an order of magnitude higher than on average. The solar wind ram pressure and the Alfvén Mach number are found to be the dominant factors affecting these changes in the magnetosphere, with the interplanetary magnetic field (IMF) direction and the IMF magnetic pressure playing a small but likely nonnegligible role.

Extremely field‐aligned cool electrons in the dayside outer magnetosphere

AbstractFor 200 days in 2016 while Time History of Events and Macroscale Interactions during Substorms D (THEMIS‐D) was in the dayside, equatorial magnetosphere, its electron energy coverage was modified such that the first 15 energy steps covered the range of 1–30 eV and 16 steps covered energies to 30 keV. These measurements were free of backgrounds from photoelectrons, secondaries, or ionospheric plasma plumes. Three energy bands of electrons were observed: cold electrons having energies below 1 eV (plasmaspheric plumes measured by the spacecraft potential); cool electrons, defined as electrons having energies of 1–25 eV; and hot electrons having energies of 25 eV to 30 keV. The cool electron fluxes at fixed radial distances varied by an order of magnitude from one orbit to the next. These fluxes often increased with increasing radial distance, suggesting an external source. They were extremely field aligned, having pitch angle ratios (flux at 0–20° and 160–180° divided by the flux at 80–100°) greater than 100. Evidence is presented that they resulted from cusp electrons moving from open to closed magnetospheric field lines due to their E × B/B2 drift. They constituted the majority of the electron energy density at such times and places. They were not associated with magnetopause reconnection because they were not observed at the magnetopause, but they were observed as far as 3 RE inside of it. Their occurrence probability in the outer magnetosphere was ~50% in June and ~10% in September, suggesting a dayside source attributed to the tilt of the northern cusp toward the Sun during the summer.

On the contribution of thermal excitation to the total 630.0 nm emissions in the northern cusp ionosphere

AbstractDirect impact excitation by precipitating electrons is believed to be the main source of 630.0 nm emissions in the cusp ionosphere. However, this paper investigates a different source, 630.0 emissions caused by thermally excited atomic oxygen O(1D) when high electron temperature prevail in the cusp. On 22 January 2012 and 14 January 2013, the European Incoherent Scatter Scientific Association (EISCAT) radar on Svalbard measured electron temperature enhancements exceeding 3000 K near magnetic noon in the cusp ionosphere over Svalbard. The electron temperature enhancements corresponded to electron density enhancements exceeding 1011 m−3 accompanied by intense 630.0 nm emissions in a field of view common to both the EISCAT Svalbard radar and a meridian scanning photometer. This offered an excellent opportunity to investigate the role of thermally excited O(1D) 630.0 nm emissions in the cusp ionosphere. The thermal component was derived from the EISCAT Radar measurements and compared with optical data. For both events the calculated thermal component had a correlation coefficient greater than 0.8 to the total observed 630.0 nm intensity which contains both thermal and particle impact components. Despite fairly constant solar wind, the calculated thermal component intensity fluctuated possibly due to dayside transients in the aurora.

Asymmetrical response of dayside ion precipitation to a large rotation of the IMF

AbstractWe have carried out global magnetohydrodynamics (MHD) simulations together with large‐scale kinetic simulations to investigate the response of the dayside magnetospheric ion precipitation to a large rotation (135°) of the interplanetary magnetic field (IMF). The study uses simplified global MHD model (no dipole tilt and constant ionospheric conductance) and idealized solar wind conditions where the IMF rotates smoothly from a southward toward a northward direction (BX = 0) to clearly identify the effects of the impact of the discontinuity on the magnetopause. Results of the global simulations reveal that a strong north‐south asymmetry develops in the pattern of precipitating ions during the interaction of the IMF rotation with the magnetopause. For a counterclockwise IMF rotation from its original southward direction (BY < 0), a spot of high‐energy particle injections occurs in the Northern Hemisphere but not in the Southern Hemisphere. The spot moves poleward and dawnward as the interacting field rotates. In that case, reconnection is found close to the poleward edge of the northern cusp, while it occurs farther tailward in the Southern Hemisphere. Tracing magnetic field lines shows an asymmetry in the tilt of the cusps and indicates that the draping and subsequent double reconnection of newly opened field lines from the Southern Hemisphere over the dayside magnetosphere cause the symmetry breaking. The reverse north‐south asymmetry is found for a clockwise IMF rotation from its original southward direction (BY > 0). Trends observed in the ion dispersions predicted from the simulations are in good agreement with Cluster observations of the midaltitude northern cusp, which motivated the study.

Strong ionospheric field‐aligned currents for radial interplanetary magnetic fields

AbstractThe present work has investigated the configuration of field‐aligned currents (FACs) during a long period of radial interplanetary magnetic field (IMF) on 19 May 2002 by using high‐resolution and precise vector magnetic field measurements of CHAMP satellite. During the interest period IMF By and Bz are weakly positive and Bx keeps pointing to the Earth for almost 10 h. The geomagnetic indices Dst is about −40 nT and AE about 100 nT on average. The cross polar cap potential calculated from Assimilative Mapping of Ionospheric Electrodynamics and derived from DMSP observations have average values of 10–20 kV. Obvious hemispheric differences are shown in the configurations of FACs on the dayside and nightside. At the south pole FACs diminish in intensity to magnitudes of about 0.1 μA/m2, the plasma convection maintains two‐cell flow pattern, and the thermospheric density is quite low. However, there are obvious activities in the northern cusp region. One pair of FACs with a downward leg toward the pole and upward leg on the equatorward side emerge in the northern cusp region, exhibiting opposite polarity to FACs typical for duskward IMF orientation. An obvious sunward plasma flow channel persists during the whole period. These ionospheric features might be manifestations of an efficient magnetic reconnection process occurring in the northern magnetospheric flanks at high latitude. The enhanced ionospheric current systems might deposit large amount of Joule heating into the thermosphere. The air densities in the cusp region get enhanced and subsequently propagate equatorward on the dayside. Although geomagnetic indices during the radial IMF indicate low‐level activity, the present study demonstrates that there are prevailing energy inputs from the magnetosphere to both the ionosphere and thermosphere in the northern polar cusp region.

Cusp observation at Saturn's high-latitude magnetosphere by the Cassini spacecraft.

We report on the first analysis of magnetospheric cusp observations at Saturn by multiple in situ instruments onboard the Cassini spacecraft. Using this we infer the process of reconnection was occurring at Saturn's magnetopause. This agrees with remote observations that showed the associated auroral signatures of reconnection. Cassini crossed the northern cusp around noon local time along a poleward trajectory. The spacecraft observed ion energy-latitude dispersions—a characteristic signature of the terrestrial cusp. This ion dispersion is “stepped,” which shows that the reconnection is pulsed. The ion energy-pitch angle dispersions suggest that the field-aligned distance from the cusp to the reconnection site varies between ∼27 and 51 RS. An intensification of lower frequencies of the Saturn kilometric radiation emissions suggests the prior arrival of a solar wind shock front, compressing the magnetosphere and providing more favorable conditions for magnetopause reconnection.Key PointsWe observe evidence for reconnection in the cusp plasma at SaturnWe present evidence that the reconnection process can be pulsed at SaturnSaturn's cusp shows similar characteristics to the terrestrial cusp

On sacred ground: the political performance of religious responsibility

April 2012: In Dambulla, a bustling market town built around a crossroads on the northern cusp of Sri Lanka's central province, a mosque was attacked by a procession of protestors led by the chief priest of the nearby Buddhist temple. Ostensibly the protest was against the presence of the mosque on the grounds that it had been built in an exclusively Buddhist ‘sacred area’. Beginning with an empirical account of the attack on the Dambulla mosque, this paper argues that the preservation of what is deemed to be ‘sacred’ in Sri Lanka provides an effective idiom through which certain religious figures can intelligibly articulate political claims whilst maintaining critical distance from the dirty world of ‘Politics’. Corollary to this, and drawing on two years of ethnographic fieldwork in Dambulla, the paper explores the various different meanings of politics locally: highlighting the interplay of everyday politicking and high-profile political performance.

Observations of Mercury's northern cusp region with MESSENGER's Magnetometer

The magnetic cusp of a planetary magnetosphere allows solar wind plasma to gain access to the planet's magnetosphere and, for Mercury, the surface. From measurements by the MESSENGER Magnetometer we have characterized the magnetic field in the northern cusp region of Mercury. The first six months of orbital measurements indicate a mean latitudinal extent of the cusp of ∼11°, and a mean local time extent of 4.5 hrs, at spacecraft altitudes. From the average magnetic pressure deficit in the cusp, we estimate that (1.1 ± 0.6) × 1024 protons s−1 bombard the surface over an area of (5.2 ± 1.6) × 1011 m2near the northern cusp. Plasma pressures in the cusp are 40% higher when the interplanetary magnetic field (IMF) is anti‐sunward than when it is sunward. The influence of the IMF direction does not overcome the north‐south asymmetry of Mercury's internal field, and particle flux to the surface near the southern cusp is predicted to be a factor of 4 greater than in the north. The higher particle flux impacting the surface in the south should lead to a greater exospheric source from the south and a higher rate of space weathering than in the area of the northern cusp.

Energetic ions near the dayside magnetopause reconnection site: Implications for energization sources

During a magnetopause crossing by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, signatures of magnetic reconnection and of accelerated flows were observed. The maximum magnetic shear model of the reconnection X-line suggests that the spacecraft passed very close to the reconnection line. At the inner edge of the low latitude boundary layer (LLBL), accelerated ions are observed, which are magnetically connected to the quasi-parallel bow shock and travel towards the northern cusp region. However, a short time later the THEMIS satellite crossed the reconnection line, onto field lines, which map out to the quasi-perpendicular bow shock (or, the weak quasi-parallel bow shock very far downstream). At these times there is no significant difference in the energetic ion flux component observed coming from the shock versus from the cusp. These signatures suggest that, at least in some cases, energetic particles observed in the cusp regions originate at the quasi-parallel bow shock.

Inter-hemispheric asymmetry of dependence of the cusp location on dipole tilt during northward IMF conditions

Abstract. A data set of the Cluster cusp crossings over a 5-year period is studied for the interhemispheric comparison of the dipole tilt angle effect on the latitude of the mid-altitude cusp. The result shows that the dipole tilt angle has a clear control of the cusp latitudinal location. Although, the northern cusp moves 0.054° ILAT for every 1° increase in the dipole tilt angle at the mean altitude of 5.2 RE, the southern cusp moves 0.051° ILAT for every 1° increase in the dipole tilt angle at the mean altitude of 6.6 RE. The northern cusp dependence agrees with the trend formed by other observations of different satellites for different altitudes, whereas the southern cusp does not. We therefore suggest that there is an inter-hemispheric difference in the dipole tilt angle dependence of cusp, latitudinal location, which has an impact on other observations of different satellites in different altitudes.

Interhemispheric comparison of GPS phase scintillation at high latitudes during the magnetic-cloud-induced geomagnetic storm of 5–7 April 2010

Abstract. Arrays of GPS Ionospheric Scintillation and TEC Monitors (GISTMs) are used in a comparative scintillation study focusing on quasi-conjugate pairs of GPS receivers in the Arctic and Antarctic. Intense GPS phase scintillation and rapid variations in ionospheric total electron content (TEC) that can result in cycle slips were observed at high latitudes with dual-frequency GPS receivers during the first significant geomagnetic storm of solar cycle 24 on 5–7 April 2010. The impact of a bipolar magnetic cloud of north-south (NS) type embedded in high speed solar wind from a coronal hole caused a geomagnetic storm with maximum 3-hourly Kp = 8- and hourly ring current Dst = −73 nT. The interhemispheric comparison of phase scintillation reveals similarities but also asymmetries of the ionospheric response in the northern and southern auroral zones, cusps and polar caps. In the nightside auroral oval and in the cusp/cleft sectors the phase scintillation was observed in both hemispheres at about the same times and was correlated with geomagnetic activity. The scintillation level was very similar in approximately conjugate locations in Qiqiktarjuaq (75.4° N; 23.4° E CGM lat. and lon.) and South Pole (74.1° S; 18.9° E), in Longyearbyen (75.3° N; 111.2° E) and Zhongshan (74.7° S; 96.7° E), while it was significantly higher in Cambridge Bay (77.0° N; 310.1° E) than at Mario Zucchelli (80.0° S; 307.7° E). In the polar cap, when the interplanetary magnetic field (IMF) was strongly northward, the ionization due to energetic particle precipitation was a likely cause of scintillation that was stronger at Concordia (88.8° S; 54.4° E) in the dark ionosphere than in the sunlit ionosphere over Eureka (88.1° N; 333.4° E), due to a difference in ionospheric conductivity. When the IMF tilted southward, weak or no significant scintillation was detected in the northern polar cap, while in the southern polar cap rapidly varying TEC and strong phase scintillation persisted for many hours. This interhemispheric asymmetry is explained by the difference in the location of solar terminator relative to the cusps in the Northern and Southern Hemisphere. Solar terminator was in the immediate proximity of the cusp in the Southern Hemisphere where sunlit ionospheric plasma was readily convected into the central polar cap and a long series of patches was observed. In contrast, solar terminator was far poleward of the northern cusp thus reducing the entry of sunlit plasma and formation of dense patches. This is consistent with the observed and modeled seasonal variation in occurrence of polar cap patches. The GPS scintillation and TEC data analysis is supported by data from ground-based networks of magnetometers, riometers, ionosondes, HF radars and all-sky imagers, as well as particle flux measurements by DMSP satellites.

Cluster observations of bow shock energetic ion transport through the magnetosheath into the cusp

[1] The observation of energetic particles by polar orbiting satellites in the magnetospheric cusp resulted in a controversy about their source region. It has been suggested that these cusp energetic particles (CEP) with significant fluxes from magnetosheath energies up to several hundred keV/e are accelerated locally in the cusp by the turbulence found in cusp diamagnetic cavities (CDC). As an alternative to the local acceleration region, the quasi-parallel shock is successful as a source region for CEP events. Energetic ions accelerated at the bow shock can be transported downstream and enter the cusp along newly reconnected field lines. Composition and energy spectra of these CEP events resemble those of bow shock energetic diffuse ions. This study investigates a northern cusp pass by the Cluster satellites that encountered two CDCs with CEP ions. We use recently developed techniques to determine the location of the reconnection site at the magnetopause, draping interplanetary magnetic field lines over the magnetopause and mapping those field lines back into the solar wind to show magnetic connection of the cusp regions, Earth's bow shock, and upstream region. Energetic ions are also observed outside the magnetopause in the boundary layer streaming from the quasi-parallel shock toward the cusp which supports an outside source region for CEP ions.

High kinetic energy density jets in the Earth’s magnetosheath: A case study

We study several high kinetic energy density jets observed during a traversal of the dayside magnetosheath by the Cluster spacecraft on March 17, 2001, at various distances from the magnetopause, generally characterised by anomalously high values of the local magnetosonic Mach number. We concentrate on two jets observed just outside the magnetopause, the first almost parallel to the GSM x axis and the second directed northward–tailward along the nominal magnetopause surface. We present evidence that none of them can be ascribed to magnetic reconnection at the magnetopause and show that the magnetopause is severely deformed by the jets, so that its local normal forms an angle of 97° with the quiet time magnetopause normal. On these grounds, we suggest that the indentation of the magnetopause is caused by an anti-sunward jet ramming into the magnetopause slightly equatorward of the northern cusp and that the northward–tailward jet is the result of its reflection at the deformed magnetopause. Finally, we briefly discuss our results by comparing them with past studies of events which in some way recall the one analysed herein.

Cluster observations of high-altitude cusp during multiple fast-turning IMF

Using coordinated observations from the Cluster, DMSP and ESR radar, high-altitude cusp was investigated for a time period characterized by four fast reversals of the IMF B Z. On February 12, 2003, Cluster was flying sunward at duskside over the Northern Hemisphere, across the high-altitude northern cusp twice. Both the two crossings are during steady northward IMF interval, whereas ion fluxes detected during the time period are quite different. The cusp in the first crossing is a “typical” northward-IMF one, with short-lived ion injections of evident “reversed” dispersion signatures. In the second crossing, however, the cusp appears to be a region reminiscent of the Stagnant Exterior Cusp (SEC). In addition, evident upward-flowing O+ ion bursts, possibly original from polar ionosphere, are also detected during the second cusp crossing, together with enhancement of trapped magnetospheric-like electron fluxes. We suggest that the SEC-like cusp is likely on newly closed field lines, which were first reconnected (being opened) at low latitudes and later reconnected again (being closed) poleward of the northern cusp.