Horizontal Plasma Flows Research Articles

The magnetically controlled ionopause boundary observed by LPW onboard MAVEN within magnetic pile-up region of Mars

We have analysed plasma and magnetic field data obtained from 627 orbits of Mars Atmosphere and Volatile Evolution (MAVEN) that occurred during October 2014, April 2015, September 2017 and May 2018, when the spacecraft was crossing the magnetic pile-up boundary of Mars. Two broad peaks and a drop are observed in this magnetic field region. The magnetic cavity boundary lies between the ionosphere and the magnetic pile-up boundary. A steep ionopause-like boundary is observed in 24 profiles of 627 orbits, when MAVEN was passing from the magnetic pile-up region during the daytime ionosphere in presence of horizontal magnetic field of high strength, while their night side ionosphere did not show such boundary in presence of horizontal magnetic field of low strength. These profiles are unique, which have not been reported earlier. We propose that the horizontal magnetic field can form such ionopause within the magnetic pile-up boundary during the daytime if time and location of the magnetic anomaly coincide with the ion and electron density measurements. The nighttime ionosphere is thought to be produced within the magnetic pile-up boundary due to transportation of plasma from dayside to nightside across the terminator by a horizontal plasma flow velocity as has been predicted by the previous studies.

A statistical study of space hurricanes in the Northern Hemisphere

The space hurricane is a newly discovered large-scale three-dimensional magnetic vortex structure that spans the polar ionosphere and magnetosphere. At the height of the ionosphere, it has a strong circular horizontal plasma flow with a nearly zero-flow center and a coincident cyclone-shaped aurora caused by strong electron precipitation associated with intense upward magnetic field-aligned currents. By analyzing the long-term optical observation onboard the Defense Meteorological Satellite Program (DMSP) F16 satellite from 2005 to 2016, we found that space hurricanes in the Northern Hemisphere occur in summer and have a maximum occurrence rate in the afternoon sector around solar maximum. In particular, space hurricanes are more likely to occur in the dayside polar cap at magnetic latitudes greater than 80°, and their MLT (magnetic local time) dependence shows a positive relationship with the IMF (interplanetary magnetic field) clock angle. We also found that space hurricanes occur mainly under dominant positive IMF By and Bz and negative Bx conditions. It is suggested that the stable high-latitude lobe reconnection, which occurs under the conditions of a large Earth’s dipole tilt angle and high ionosphere conductivity in summer, should be the formation mechanism of space hurricanes. The result will give a better understanding of the solar wind–magnetosphere–ionosphere coupling process under northward IMF conditions.

A space hurricane over the Earth\u2019s polar ionosphere

In Earth’s low atmosphere, hurricanes are destructive due to their great size, strong spiral winds with shears, and intense rain/precipitation. However, disturbances resembling hurricanes have not been detected in Earth’s upper atmosphere. Here, we report a long-lasting space hurricane in the polar ionosphere and magnetosphere during low solar and otherwise low geomagnetic activity. This hurricane shows strong circular horizontal plasma flow with shears, a nearly zero-flow center, and a coincident cyclone-shaped aurora caused by strong electron precipitation associated with intense upward magnetic field-aligned currents. Near the center, precipitating electrons were substantially accelerated to ~10 keV. The hurricane imparted large energy and momentum deposition into the ionosphere despite otherwise extremely quiet conditions. The observations and simulations reveal that the space hurricane is generated by steady high-latitude lobe magnetic reconnection and current continuity during a several hour period of northward interplanetary magnetic field and very low solar wind density and speed.

Evidence of vertical coupling: meteorological storm Fabienne on 23 September 2018 and its related effects observed up to the ionosphere

Abstract. A severe meteorological storm system on the frontal border of cyclone Fabienne passing above central Europe was observed on 23–24 September 2018. Large meteorological systems are considered to be important sources of the wave-like variability visible/detectable through the atmosphere and even up to ionospheric heights. Significant departures from regular courses of atmospheric and ionospheric parameters were detected in all analyzed datasets through atmospheric heights. Above Europe, stratospheric temperature and wind significantly changed in coincidence with fast frontal transition (100–110 km h−1). Zonal wind at 1 and 0.1 hPa changes from the usual westward before the storm to eastward after the storm. With this change are connected changes in temperature where at 1 hPa the analyzed area is colder and at 0.1 hPa warmer. Within ionospheric parameters, we have detected significant wave-like activity occurring shortly after the cold front crossed the observational point. During the storm event, both by Digisonde DPS-4D and continuous Doppler sounding equipment, we have observed strong horizontal plasma flow shears and time-limited increase plasma flow in both the northern and western components of ionospheric drift. The vertical component of plasma flow during the storm event is smaller with respect to the corresponding values on preceding days. The analyzed event of an exceptionally fast cold front of cyclone Fabienne fell into the recovery phase of a minor–moderate geomagnetic storm observed as a negative ionospheric storm at European mid-latitudes. Hence, ionospheric observations consist both of disturbances induced by moderate geomagnetic storms and effects originating in convective activity in the troposphere. Nevertheless, taking into account a significant change in the global circulation pattern in the stratosphere, we conclude that most of the observed wave-like oscillations in the ionosphere during the night of 23–24 September can be directly attributed to the propagation of atmospheric waves launched on the frontal border (cold front) of cyclone Fabienne. The frontal system acted as an effective source of atmospheric waves propagating upward up to the ionosphere.

A Small Pore Observed with a 1.6 m Telescope

A small pore was observed with the New Solar Telescope of 1.6 m aperture at Big Bear Solar Observatory. At a wavelength of 705.68 nm, a diffraction limit of 0.09″ was reached and approximately 700 small structures were analyzed. The achieved high resolution has revealed details about the dynamics of small structures in and around the pore. The small structures either reside in the pore, enter the pore from the vicinity, or dip in and out of the pore. Their sizes are similar, ≈ 0.2″, but their observed oscillations do not show power peaks at the same periods. Horizontal plasma flows were suppressed in the pore by 38 % for average velocities, and 65 % for maximum velocities, compared to the quiet Sun.

The Horizontal Component of Photospheric Plasma Flows During the Emergence of Active Regions on the Sun

The dynamics of horizontal plasma flows during the first hours of the emergence of active region magnetic flux in the solar photosphere have been analyzed using SOHO/MDI data. Four active regions emerging near the solar limb have been considered. It has been found that extended regions of Doppler velocities with different signs are formed in the first hours of the magnetic flux emergence in the horizontal velocity field. The flows observed are directly connected with the emerging magnetic flux; they form at the beginning of the emergence of active regions and are present for a few hours. The Doppler velocities of flows observed increase gradually and reach their peak values 4 – 12 hours after the start of the magnetic flux emergence. The peak values of the mean (inside the ± 500 m s−1 isolines) and maximum Doppler velocities are 800 – 970 m s−1 and 1410 – 1700 m s−1, respectively. The Doppler velocities observed substantially exceed the separation velocities of the photospheric magnetic flux outer boundaries. The asymmetry was detected between velocity structures of leading and following polarities. Doppler velocity structures located in a region of leading magnetic polarity are more powerful and exist longer than those in regions of following polarity. The Doppler velocity asymmetry between the velocity structures of opposite sign reaches its peak values soon after the emergence begins and then gradually drops within 7 – 12 hours. The peak values of asymmetry for the mean and maximal Doppler velocities reach 240 – 460 m s−1 and 710 – 940 m s−1, respectively. An interpretation of the observable flow of photospheric plasma is given.

Investigating the southern daytime midlatitude trough's relation with the daytime Weddell Sea Anomaly during equinoxes

A detailed study with statistical investigations was conducted on the morning topside ionosphere for the 1996/1997 equinoctial period utilizing Defense Meteorological Satellite Program data and coupled thermosphere‐ionosphere plasmasphere wind simulations. Highly structured plasma densities, including a daytime Weddell Sea Anomaly (WSA; about 0900 LT) over the southeastern Pacific and localized high‐latitude plasma enhancements (about 0800 LT) in the Australian sector, were tracked. Poleward and equatorward winds, respectively, were strongest there. High‐resolution regional surface maps and field‐aligned profiles tracked their thermal structures, O+ and H+ composition, vertical drifts (VZ, m/s) and horizontal plasma flows {FY; [i+/(cm2s)]}. A daytime heavy‐ion (Ni‐O+) stagnation trough [(54.5 ± 4.5)°S; geomagnetic] appeared equatorward of both the WSA [60.0 ± 4.0)°S (center), 185–310°E; geographic] and these enhancements [>(62.0 ± 3.5)°S; 30–180°E]. Meanwhile, the plasmapause developed poleward of the WSA and equatorward of these enhancements. Plasma convection became significantly increased by enhanced electron temperatures nearby upflows underlying localized plasma enhancements. Upflow velocities, modeled as about 694.5 m/s, shows close similarities to the actual direct measurements of about 700 m/s. Both slow and enhanced plasma convections promoted stagnation events and trough formation. However, regular trough features became significantly altered by the development of a downward dipping poleward wall in the WSA region and by the absence of regular trough walls in the region of localized plasma enhancements. Statistically, the trough and localized plasma enhancement locations varied linearly with magnetic activity and showed an equatorward movement with increasing activity. A rate of (0.026 ± 0.002)°S/nT for the trough and (0.067 ± 0.003)°S/nT for the localized plasma enhancements were modeled. This higher (approximately two times) rate of movement indicates the localized plasma enhancements' more responsiveness.

Investigating the relationships among the South Atlantic Magnetic Anomaly, southern nighttime midlatitude trough, and nighttime Weddell Sea Anomaly during southern summer

This study utilized the multi‐instrument data of the Defense Meteorological Satellite Program to investigate the evening/nighttime topside ionosphere during the 1996/1997 southern summer. A series of regional surface maps were constructed and permitted the tracking of the topside ionosphere's plasma density features, plasma composition, thermal structures, and vertical and horizontal plasma flows. These maps tracked a complete nighttime Weddell Sea Anomaly (WSA) and strong horizontal plasma flows that registered the high‐conductivity regions of the South Atlantic Magnetic Anomaly (SAMA). These regions developed over the southeastern Pacific, just equatorward of the WSA, and over the South Atlantic. A heavy‐ion stagnation trough developed poleward of the SAMA affected regions. Thus, the trough appeared on the WSA's equatorward side. During periods of increasing magnetic activity, the plasmapause was the WSA's poleward boundary. A statistical study modeled the trough's magnetic activity dependence and revealed a strong east‐west hemispherical difference that was due to the SAMA effects. When the AE6 was 0 nT, the trough appeared at (57.49 ± 2.82)°S (geomagnetic) over the southwestern hemisphere. Owing to the SAMA's special electrodynamic effects, the trough developed at lower latitudes, (42.39 ± 3.04)°S, over the southeastern hemisphere. Meanwhile, the plasmapause occurred at ∼(62.5 ± 4)°S, and the WSA's peak appeared at ∼(56.2 ± 4)°S. Hence, there was a ∼20° (lat) separation between the trough and the plasmapause over the southeastern hemisphere. Between 210°E and 330°E (geographic), the WSA filled this gap. With increasing magnetic activity, the trough in the SAMA affected regions moved poleward at a rate of (0.0157 ± 0.004)°S/nT. Elsewhere, it had a (0.0196 ± 0.002)°S/nT equatorward movement.

Photospheric Magnetic Activities Responsible for Soft X‐Ray Pointlike Microflares. I. Identifications of Associated Photospheric/Chromospheric Activities

By combining Yohkoh soft X-ray images with high-resolution magnetograms simultaneously obtained at La Palma, we studied photospheric magnetic signatures responsible for soft X-ray microflares (active-region transient brightenings). In order to have a reliable correspondence between the photosphere and the corona, we studied 16 pointlike transient brightenings with X-ray source size less than 10'' occurring during periods when the seeing was excellent at La Palma, although a lot of transient brightenings were in forms of multiple- or single-loop structures. In half of the studied events, small-scale emergences of magnetic flux loops are found in the vicinity of the transient brightenings. Six events of that half show that a small-scale flux emergence accompanies the X-ray brightening 5-30 minutes prior to its onset. In the other half of the studied events, no apparent evolutionary change of magnetic flux elements is found associated with the transient brightenings. Many of these events are found in rather strong magnetic fields, such as sunspots and pores, implying that small-scale changes of magnetic flux are obscured or suppressed by strong magnetic fields. The horizontal plasma flows derived from local cross-correlation tracking of granules in continuum images are suppressed at the feet of some X-ray transient brightenings.

Two-Sided-Loop-Type X-ray Jets and Metric Radio Bursts

We have searched for nonthermal radio signatures in the form of metric type III bursts in conjunction with two-sided-loop-type X-ray jets observed by the Yohkoh/SXT experiment. We have found no evidence of type III bursts in association with this particular type of X-ray jets in contrast to the positive evidence of type III's in association with anemone-type X-ray jets. This result is consistent with the simulation results of Yokoyama and Shibata (1995), which show that anemone-type jets are produced by vertical/oblique plasma flow whereas the two-sided-loop-type jets are produced by horizontal plasma flow.

Horizontal plasma flow at midlatitudes: More mechanisms and the interpretation of observations

The midlatitude ionosphere has been known for decades to exhibit correlated (or anticorrelated, depending on the coordinate system) perpendicular and parallel meridional ion velocities. This correlation is indicative of horizontal plasma flow, despite the highly magnetized character of the midlatitude F region. This coupling of the perpendicular and parallel dynamics is a consequence of extremely weak atmosphere‐ionosphere coupling and the strong tendency for the neutral atmosphere to flow horizontally. Three physical mechanisms have previously been identified as effecting this coupling: ion drag, the F region dynamo, and enhanced gravity‐driven diffusion. We point out some difficulties with one of these processes and present two additional mechanisms, wind‐driven ionospheric equilibrium and gravity wave seeding of plasma instabilities, which can also result in horizontal plasma flow. Although these additional mechanisms further complicate the observational distinction between the possible mechanisms, we argue that an appropriately clustered array of instruments can still distinguish which mechanism is responsible for any particular geophysical occurrence.

Freja observations of narrow inverted‐V electron precipitation by the Two‐Dimensional Electron Spectrometer

Observations of several steady, ∼1–5 km scale inverted‐V's by the Freja F7 Two‐dimensional Electron Spectrometer (TESP) are described. Such narrow structures are not common, but provide an opportunity for observing edge precipitation and the movement of arcs through the plasma. Velocities of inverted‐V potential structures in the plasma frame can be derived from the observed time delay between precipitating and mirrored electrons. These velocities determine whether horizontal plasma flow into a potential structure can provide the source population for arc edge precipitation. It is found that the intensity of the arc edge precipitation is not associated in any expected way with this velocity, indicating another source of plasma for arc edge precipitation is required.

Theory for modeling the equatorial evening ionosphere and the origin of the shear in the horizontal plasma flow

Companion papers in this series present (1) the role of equatorial E region postsunset ionosphere, (2) the origin of horizontal plasma shear flow in the postsunset equatorial ionosphere (this paper), (3) the Coloured Bubbles experiments results, and (4) computer simulations of artificial initiation of plasma density depletions (bubbles) in the equatorial ionosphere. Within this paper, equations describing the time evolution of the equatorial ionosphere are developed using flux tube integrated and flux tube weighted quantities which model the chemistry, dynamics, and electrodynamics of the equatorial ionosphere. The resulting two‐dimensional set of equations can be used to investigate equatorial electric fields neglecting small‐scale phenomena (λ < 1 km). An immediate result derived from the integrated current equations is an equation describing the physics of the shear in the horizontal flow of the equatorial plasma during the evening hours. The profile of the horizontal flow has three important contributing terms relating to the neutral wind dynamo, Hall conduction, and the equatorial electrojet current divergence. Using a one‐dimensional model of the velocity shear equation and the integrated ionospheric transport equations, a time history of the development of the shear feature during the postsunset hours is presented. The one‐dimensional model results are compared to the velocity shear measurements from the Coloured Bubbles experiments.

A two‐dimensional kinematic dynamo model of the ionospheric magnetic field at Venus

The ionosphere of Venus was observed by the magnetometer on the Pioneer Venus Orbiter (PVO) to be permeated by a large‐scale magnetic field for conditions of high solar wind dynamic pressure. The ionospheric conductivity is very high above 170 km and magnetic flux is carried by the plasma flow both vertically downward and in an antisunward direction. Ohmic dissipation of the electric current allows the magnetic field to diffuse for altitudes below 150–170 km. A number of one‐dimensional kinematic dynamo and MHD models have been developed to explain the observed vertical magnetic field profiles, but did not include the effects of horizontal transport. We have constructed a two‐dimensional kinematic dynamo model of the dayside ionospheric magnetic field at Venus in which the 2‐D magnetic induction equation (i.e., magnetic diffusion‐convection equation) is numerically solved using specified plasma velocities. The vertical velocity profile is taken from the one‐dimensional MHD model of Shinagawa and Cravens [1988] and the horizontal velocities are adapted from published PVO retarding potential analyzer results [Knudsen et al., 1982]. We demonstrate that the basic features of the vertical magnetic field profile, such as the presence of a magnetic layer at 170 km, remain unaltered by horizontal flow effects. However, we also show that horizontal plasma flow can strongly affect the magnetic field for altitudes above 300 km.

A simple spectral model of the dynamics of the Venus ionosphere

A two‐dimensional model of the ionosphere of Venus has been constructed by expanding pertinent quantities in Legendre polynomials. The model is simplified by including only a single ion species (O+). Horizontal plasma flow velocity and plasma density have been calculated as a coupled system. The calculated plasma flow velocity is found to be in good agreement with observations and the results of earlier studies. Solar zenith angle dependence of plasma density, particularly on the nightside, shows some features which differ from results of earlier studies and observed values. Effects of raising or lowering the ionopause height and changing the nightside neutral atmosphere have been discussed.

Venus' nighttime horizontal plasma flow, ‘magnetic congestion’, and ionospheric hole production

A persistent feature of the nightside Venus ionosphere is the existence of localized magnetic field aligned columns of depleted ionization—i.e., ‘holes’—in which the radial component of the field is enhanced. By analogy to the earth's auroral regions it has been inferred that parallel electric fields can rapidly deplete the ionization. Another, complementary plasma loss mechanism at night is downward transport into the lower ionosphere where chemical depletion occurs. A simple rectilinear two dimensional MHD model is used to explore the effects of field aligned plasma loss and cooling on a dense plasma convecting across a weak magnetic field. By parameterizing field aligned variations and explicitly solving for cross variations, it is shown that the abrupt horizontal enhancements of the vertical magnetic field as well as abrupt decreases of the plasma density to very low values—the two characteristics of a hole—can be produced in the presence of field aligned losses. The buildup of the magnetic field occurs in conjunction with a deceleration of the horizontal flow. The horizontal decrease in plasma density occurs abruptly when the magnetic field intensifies to such a level that the associated horizontal transport time becomes smaller than the field aligned loss time; this process is referred to as ‘magnetic congestion’.