Mass Density Enhancement Research Articles

Two‐Dimensional Local Modeling of Thermospheric Heating and Neutral Mass Density Enhancement Driven by Alfvén Waves

AbstractIn the cusp region, a significantly enhanced thermospheric mass density is commonly observed around 400 km altitude. Despite a number of studies, the enhancement mechanism has not been fully characterized. In order to determine how the Joule heating and resultant mass density enhancements are generated in the F region of the ionosphere during a few hours after the Alfvén resonator modes are set up, we have developed a new efficient method to calculate Alfvén waves. In this method, the Fourier transform was used, and Alfvén waves were solved as frequency‐domain boundary value problems. We employed a two‐dimensional local model and performed five modeling runs. The result from the modeling runs shows that the Alfvén resonator modes generate significant neutral upwelling at ∼300 km altitude, which creates a “cell” of the neutral mass density enhancement at altitudes centered between 350 and 400 km. This cell becomes evident roughly 1 hr after the Alfvén resonator modes are set up, and this region continues to exist stably for 2 more hours. A fractional mass density enhancement at 400 km altitude 3 hr after the Alfvén resonator modes having an Alfvénic field‐aligned current of 20 at the top boundary are set up reaches ∼30%, which is consistent with the result obtained from satellite observations. In terms of the Poynting flux, this corresponds to ∼20 mWm−2, which is also consistent with previous satellite observations.

Unexpected space weather causing the reentry of 38 Starlink satellites in February 2022

The accidental reentry of 38 Starlink satellites occurred in early February 2022, associated with the occurrence of moderate magnetic storms. A poorly understood structure of Coronal Mass Ejections (CMEs) caused the magnetic storms at unexpected timing. Therefore, a better understanding of minor CME structures is necessary for the modern space weather forecast. During this event, the “up to 50%” enhancement of air drag force was observed at ~200 km altitude, preventing the satellites’ safety operations. Although the mass density enhancement predicted by the NRLMSIS2.0 empirical model is less than 25% under the present moderate magnetic storms, the real-time GAIA simulation showed a mass density enhancement of up to 50%. Further, the real-time GAIA simulation suggests that the actual thermospheric disturbances at 200 km altitude may occur with larger amplitude in a broader area than previously thought.

Time-dependent responses of the neutral mass density to fixed magnetospheric energy inputs into the cusp region in the thermosphere during a period of large IMF BY: a high-resolution two-dimensional local modeling

Thermospheric mass density values around the 400-km altitude in the cusp can be significantly enhanced as compared to regions around the cusp. To gain insights into the extent to which the magnitude of the cusp mass density enhancements can be explained by the static distributions of moderate electric field and electron precipitation typical for a period of large IMF BY, we employed a high-resolution two-dimensional local model that can represent the plasma features that are characteristic of the cusp: azimuthal ion flow and low-energy electron precipitation. We also calculated the thermospheric dynamics with and without neutral–ion drag. We found that in the calculation with this drag the obtained mass density enhancement is 10% at most, indicating that the thermospheric dynamics imposing the moderate static electric field and electron precipitation can only explain about one-third of the typical magnitude of cusp thermospheric mass density, i.e., 33%. We also found that in the calculation without neutral–ion drag the magnitude of the mass density enhancement is slightly larger than the one with the neutral–ion drag. To explain the average magnitude of the cusp mass density enhancements completely, other energy inputs such as Alfvén waves, in addition to the static distributions of electric field and electron precipitation, are needed.Graphical

The Relationship Between Large Scale Thermospheric Density Enhancements and the Spatial Distribution of Poynting Flux

AbstractLarge thermospheric neutral density enhancements in the cusp region have been examined for many years. The Challenging Minisatellite Payload (CHAMP) satellite for example has enabled many observations of the perturbation, showing that it is mesoscale in size and exists statistically over solar cycle timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine‐scale Poynting fluxes are associated with the density perturbations on a case‐by‐case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment datasets to generate global‐scale patterns of the high‐latitude and height‐integrated Poynting flux into the ionosphere, with a time resolution of 2 min. From these, average patterns are generated based on the interplanetary magnetic field orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Importantly, the lack of correlation between mesoscale height‐integrated Poynting fluxes and the cusp neutral mass density enhancement gives possible insight into other processes that may account for the discrepancy, such as energy deposition at finer scale sizes or at higher altitudes than captured.

Formation and Reconnection of Three-dimensional Current Sheets with a Guide Field in the Solar Corona

Abstract We analyze a series of three-dimensional magnetohydrodynamic numerical simulations of magnetic reconnection in a model solar corona to study the effect of the guide-field component on quasi-steady-state interchange reconnection in a pseudostreamer arcade configuration. This work extends the analysis of Edmondson et al. by quantifying the mass density enhancement coherency scale in the current sheet associated with magnetic island formation during the nonlinear phase of plasmoid-unstable reconnection. We compare the results of four simulations of a zero, weak, moderate, and a strong guide field, , to quantify the plasmoid density enhancement’s longitudinal and transverse coherency scales as a function of the guide-field strength. We derive these coherency scales from autocorrelation and wavelet analyses, and demonstrate how these scales may be used to interpret the density enhancement fluctuation’s Fourier power spectra in terms of a structure formation range, an energy continuation range, and an inertial range—each population with a distinct spectral slope. We discuss the simulation results in the context of solar and heliospheric observations of pseudostreamer solar wind outflow and possible signatures of reconnection-generated structure.

Effect of cold high pressure deformation on the properties of ex situ MgB2 wires

A substantially different behaviour was observed between MgB2 wires produced either by in situ or ex situ processing after applying the recently developed technique of cold high pressure densification (or CHPD). In contrast to in situ wires, where densification at 1.5 GPa on binary and ternary alloyed in situ MgB2 wires causes an enhancement of mass density and a strong enhancement of Jc, ex situ wires up to 2 GPa show only a negligible enhancement of the MgB2 mass density, while a considerable enhancement of Jc is still observed. In both cases, this reflects an enhancement of grain connectivity, however in ex situ wires, the enhancement of Jc is connected to the partial disruption of the oxide layer around each MgB2 powder particle, in contrast to the enhancement of Jc in situ wires, which is correlated to a smaller void fraction and a larger contact area between neighbouring grains. It is well known that Jc of ex situ wires decreases after longer exposition times when exposed to air prior to annealing; after several months Jc falls to values ≤50% of the original value. After cold pressing, we have found that Jc of the same wire exceeded the original values, even after exposing the unreacted wire for >1 year to air. A lower electrical resistivity is measured on pressed ex situ MgB2 wires, which confirms the improvement of grain connectivity due to the breakage of the oxide layers. Thus, the application of high pressure at room temperature allows us to recover the values of Jc for ex situ wires even after they have been degraded either by low quality MgB2 precursors or by prolonged ageing of the already formed wire prior to the final heat treatment. The limits of the Jc enhancement in ex situ wires are discussed.

In-Situ CHAMP Observation of Ionosphere-Thermosphere Coupling

The coupling between the ionised plasma and the neutral thermospheric particles plays an important role for the dynamics of the upper atmosphere. Significant progress in understanding the related processes has been achieved thanks to the availability of continuous accurate measurements of thermospheric parameters like mass density and wind by high resolution accelerometers on board the satellites CHAMP and GRACE. Here we present some examples of ionosphere-thermosphere coupling where CHAMP observations contributed considerably to their interpretation. We start with the derived properties of the thermosphere at altitudes around 400 km. A new aspect is the significant control of the geomagnetic field geometry on thermospheric features. Phenomena discussed in some depths are the equatorial mass density anomaly, the cusp-related mass density enhancement and the thermospheric response to magnetospheric substorms. Here we consider both the effect on the density and on the wind. A long predicted process is the wind-driven ionospheric F region dynamo. The high-resolution magnetic field measurements of CHAMP enabled for the first time a systematic study of that phenomenon considering longitudinal, local time, seasonal and solar flux dependences. Some open issues that require further investigations are mentioned at the end.

Magnetospheric seismology using multiharmonic toroidal waves observed at geosynchronous orbit

The frequencies of the harmonics of standing Alfvén waves depend on the field line distribution of mass that sustains the waves. This fact is the basis of inversion techniques to estimate the plasma mass density variation along the magnetic field from the frequencies of observed ultra‐low‐frequency waves (normal mode magnetospheric seismology). However, the technique has not been used extensively because it is difficult to accurately determine the frequencies. Wave observations from geostationary satellites provide the best chance of a successful implementation of the technique because the satellites accumulate the spectral information of the harmonics with minimal variations in frequency. We have statistically studied the frequency of multiharmonic toroidal waves detected by the GOES 5 geostationary satellite. We quantify the relationships among the harmonics using the frequency ratio fn/f3, where fn is the frequency of the nth harmonic (n = 1 for the fundamental mode). The third harmonic is chosen for the reference frequency because it is the most ubiquitous mode at GOES 5. Using all data acquired in 1986, we find that f1/f3 changes from 0.25 in the early morning to 0.22 in the afternoon, whereas the other frequency ratios are more nearly constant: f2/f3, = 0.634–0.646, f4/f3 = 1.36–1.37, and f5/f3 = 1.73–1.74. The frequencies are in general inconsistent with the power law density model. When the frequencies are used in the density modeling technique developed by Denton et al. (2001, 2004), we find an equatorial mass density enhancement in the afternoon local time sector but not on the dawnside.

Results of a new approach to determining the density function in the galactic plane - The local system

Star counts in the directions of 15 opaque CO clouds of known distance are used to determine the density function in the galactic plane of the local system of apparently enhanced stellar density. Counts were made on Palomar Sky Survey blue prints in the directions of the highest opacity portions of 15 molecular clouds. Average values of the density functions obtained are found to be significantly less than unity for clouds more distant than 800 pc regardless of galactic longitude, confirming the presence of the local mass density enhancement. Data suggest that the local system represents a density enhancement of a factor of 2 over a region of approximately 500 pc. An empirical relationship is also determined between the distance to a molecular cloud and the number of foreground stars projected on it, which may be useful in determining distances to clouds nearer than 3 kpc. The potential of the Space Telescope for complete stellar mass determinations out to a distance of 1 kpc is pointed out.