Raluca Ilie

Influence of epoch time selection on the results of superposed epoch analysis using ACE and MPA data

[1] The influence of the reference time selection when conducting a superposed epoch analysis is examined for intense geomagnetic storms at solar maximum. The events were selected according to the minimum pressure-corrected Dst, Dst*, being less than � 100 nT. Solar wind data from ACE are used, along with near-Earth data from the magnetospheric plasma analyzer (MPA) instruments on the Los Alamos National Laboratory–operated geosynchronous spacecraft. Numerous choices for the zero epoch time are used, ranging from the storm sudden commencement (SSC), the peak of the ring current enhancement (minimum Dst* slope), to the time of the storm peak (minimum Dst* value). When doing superposed epoch analysis (SEA), the choice of the time stamp can be very important; for different choices, different storm characteristics are evident in the averaged data. In the superposed ACE data we find that when using the SSC as a time reference, the SSC-related jump in solar wind parameters is very well defined, but near the storm peak, Bz does not apparently follow the well-known criteria for intense storms (Bz � � 10 nT for more than 3 h), even though this criterion is met by most of the individual storms selected for this study. When the zero epoch time is chosen near the storm peak, the jump in solar wind parameters is less distinct (and eventually lost), but the criterion for Bz is met. Regarding the MPA data, there are certain parameters that require the choice of a specific epoch time in order to produce a systematic behavior in the SEA analysis and others that are less sensitive to this choice of epoch time, since they appear to be less distinct in their temporal and spatial location. For instance, the nightside and morningside hot-ion density and temperature are main phase traits, and a zero epoch time near the peak of the ring current enhancement is required to make these features distinct.

An investigation of the magnetosphere-ionosphere response to real and idealized co-rotating interaction region events through global magnetohydrodynamic simulations

This study investigates the role of interplanetary magnetic field (IMF) Bz fluctuations periodicity in the transfer of solar wind mass and energy to the magnetosphere during the co-rotating interaction region/high-speed stream event of 10 November 2003 through global modelling simulations using the space weather modelling framework. To do so, we used both solar wind observations and a variety of idealized inputs as upstream boundary conditions, describing different solar wind configurations for which relative contribution of the peak-to-noise ratio in the input Bz power spectrum to the periodicity transfer is examined. Fast Fourier transforms of both input to and the response of the magnetosphere reveal that the transfer of IMF Bz periodicity to the magnetosphere is unaltered by other solar wind parameters, although the size of the peak-to-noise ratio of the input signal is the controlling factor that determines this transfer. The global magnetosphere simulation suggests that a threshold amount of power (peak-to-noise ratio) of approximately 10 in the input signal is needed for the magnetosphere to react to the periodicity in the input Bz, while for the cross-polar cap potential, the threshold amount is significantly smaller.

Statistical analysis of the geomagnetic response to different solar wind drivers and the dependence on storm intensity

Geomagnetic storms start with activity on the Sun that causes propagation of magnetized plasma structures in the solar wind. The type of solar activity is used to classify the plasma structures as being either interplanetary coronal mass ejection (ICME) or corotating interaction region (CIR) driven. The ICME-driven events are further classified as either magnetic cloud (MC) driven or sheath (SH) driven by the geoeffective structure responsible for the peak of the storm. The geoeffective solar wind flow then interacts with the magnetosphere producing a disturbance in near-Earth space. It is commonly believed that a SH-driven event behaves more like a CIR-driven event than a MC-driven event; however, in our analysis this is not the case. In this study, geomagnetic storms are investigated statistically with respect to the solar wind driver and the intensity of the events. We use the Hot Electron and Ion Drift Integrator (HEIDI) model to simulate the inner magnetospheric hot ion population during all of the storms classified as intense (Dstmin ≤ −100 nT) within solar cycle 23 (1996–2005). HEIDI is configured four different ways using either the Volland-Stern or self-consistent electric field and either event-based Los Alamos National Laboratory (LANL) magnetospheric plasma analyzer (MPA) data or a reanalyzed lower resolution version of the data that provides spatial resolution. Presenting the simulation results, geomagnetic data, and solar wind data along a normalized epoch timeline shows the average behavior throughout a typical storm of each classification. The error along the epoch timeline for each HEIDI configuration is used to rate the models performance. We also subgrouped the storms based on the magnitude of the minimum Dst. We found that typically the LANL MPA data provide the best outer boundary condition. Additionally, the self-consistent electric field better reproduces SH- and MC-driven events throughout most of the storm timeline, but the Volland-Stern electric field better reproduces CIR-driven events. Contrary to what we expect, examination of the HEIDI model results and solar wind data shows that SH-driven events behave more like MC-driven events than CIR-driven storms.

Plasma properties of superstorms at geosynchronous orbit: How different are they?

[1] Plasma measurements at geosynchronous orbit are examined via superposed epoch analysis for various storm categories to assess whether superstorms have an unusually altered source population for the storm-time ring current. It is found that certain characteristics of this near-Earth plasma distribution during superstorms are similar to those of moderate or intense storms, or extensions of the trends seen in these lesser storms. These similarities include the dawn sector development of cold, dense plasma. However, other characteristics are unique to superstorms, such as the existence of cold, dense plasma at dusk and midnight. It is concluded that the ring current source during superstorms is a combination of the usual storm-time characteristics as well as an unusually altered component.

A numerical study of the correspondence between paths in a causal set and geodesics in the continuum

This paper presents the results of a computational study related to the pathgeodesic correspondence in causal sets. For intervals in flat spacetimes, and in selected curved spacetimes, we present evidence that the longest maximal chains (the longest paths) in the corresponding causal set intervals statistically approach the geodesic for that interval in the appropriate continuum limit.

Assessing the role of oxygen on ring current formation and evolution through numerical experiments

Abstract We address the effect of ionospheric outflow and magnetospheric ion composition on the physical processes that control the development of the 5 August 2011 magnetic storm. Simulations with the Space Weather Modeling Framework are used to investigate the global dynamics and energization of ions throughout the magnetosphere during storm time, with a focus on the formation and evolution of the ring current. Simulations involving multifluid (with variable H+/O+ ratio in the inner magnetosphere) and single‐fluid (with constant H+/O+ ratio in the inner magnetosphere) MHD for the global magnetosphere with inner boundary conditions set either by specifying a constant ion density or by physics‐based calculations of the ion fluxes reveal that dynamical changes of the ion composition in the inner magnetosphere alter the total energy density of the magnetosphere, leading to variations in the magnetic field as well as particle drifts throughout the simulated domain. A low oxygen to hydrogen ratio and outflow resulting from a constant ion density boundary produced the most disturbed magnetosphere, leading to a stronger ring current but misses the timing of the storm development. Conversely, including a physics‐based solution for the ionospheric outflow to the magnetosphere system leads to a reduction in the cross‐polar cap potential (CPCP). The increased presence of oxygen in the inner magnetosphere affects the global magnetospheric structure and dynamics and brings the nightside reconnection point closer to the Earth. The combination of reduced CPCP together with the formation of the reconnection line closer to the Earth yields less adiabatic heating in the magnetotail and reduces the amount of energetic plasma that has access to the inner magnetosphere.

Ionospheric control of the dawn‐dusk asymmetry of the Mars magnetotail current sheet

This study investigates the role of solar EUV intensity at controlling the location of the Mars magnetotail current sheet and the structure of the lobes. Four simulation results are examined from a multifluid magnetohydrodynamic model. The solar wind and interplanetary magnetic field (IMF) conditions are held constant and the Mars crustal field sources are omitted from the simulation configuration. This isolates the influence of solar EUV. It is found that solar maximum conditions, regardless of season, result in a Venus-like tail configuration with the current sheet shifted to the –Y (dawnside) direction. Solar minimum conditions result in a flipped tail configuration with the current sheet shifted to the +Y (duskside) direction. The lobes follow this pattern, with the current sheet shifting away from the larger lobe with the higher magnetic field magnitude. The physical process responsible for this solar EUV control of the magnetotail is the magnetization of the dayside ionosphere. During solar maximum, the ionosphere is relatively strong and the draped IMF field lines quickly slip past Mars. At solar minimum, the weaker ionosphere allows the draped IMF to move closer to the planet. These lower altitudes of the closest approach of the field line to Mars greatly hinders the day-to-night flow of magnetic flux. This results in a buildup of magnetic flux in the dawnside lobe as the S-shaped topology on that side of the magnetosheath extends farther downtail. The study demonstrates that the Mars dayside ionosphere exerts significant control over the nightside induced magnetosphere of that planet.

Challenges associated with near‐Earth nightside current

Every magnetic field of solar and planetary space environments is associated with a current of differentially flowing charged particles. Electric potential patterns in geospace and near other planets are also closely linked with currents. Close to the Earth, particularly in the near-Earth nightside magnetosphere, several current systems wax and wane during periods of space weather activity. The velocity-dependent drift, energization, and loss processes in this region complicate current system evolution. There is a discrepancy about the magnitude, timing, and location of these currents, however, and this Commentary pitches the case for a concerted community effort to resolve this issue.

The outflow of ionospheric nitrogen ions: A possible tracer for the altitude-dependent transport and energization processes of ionospheric plasma

Though limited, the existing observational data set indicates that N+ is a significant ion in the ionosphere, and its concentration varies with season, time of day, solar cycle, latitude, and geomagnetic conditions. Knowledge of the differential transport of heavy vs. light ionospheric species can provide the connection between the macro-scale dynamics and micro-scale processes that govern the near-Earth space. The mass distribution of accelerated ionospheric ions reflects the source region of the low-altitude ion composition and the minor ion component can serve as a tracer of ionospheric processes since they can have a significant influence on the local plasma dynamics.

Geomagnetic disturbance intensity dependence on the universal timing of the storm peak

The role of Universal Time (UT) dependence on storm-time development has remained an unresolved question in geospace research. This study presents new insight into storm progression in terms of the UT of the storm peak. We present a superposed epoch analysis of solar wind drivers and geomagnetic index responses during magnetic storms, categorized as a function of UT of the storm peak, to investigate the dependency of storm intensity on UT. Storms with Dst minimum less than - 100 nT were identified in the 1970 - 2012 era (305 events), covering four solar cycles. The storms were classified into 6 groups based on the UT of the minimum Dst (40 to 61 events per bin), then each grouping was superposed on a timeline that aligns the time of the minimum Dst. Fifteen different quantities were considered, seven solar wind parameters and eight activity indices derived from ground-based magnetometer data. Statistical analyses of the superposed means against each other (between the different UT groupings) were conducted to determine the mathematical significance of similarities and differences in the time series plots. It was found that the solar wind parameters have no significant difference between the UT groupings, as expected. The geomagnetic activity indices, however, all show statistically significant differences with UT during the main phase and/or early recovery phase. Specifically, the 02:00 UT groupings are stronger storms than those in the other UT bins. That is, storms are stronger when the Asian sector is on the nightside (American sector on the dayside) during the main phase.