A space hurricane over the Earth's polar ionosphere

Hurricanes in Earth's lower atmosphere are characterized by a low-pressure center, strong winds and flow shears, and spiral cloud formations. While similar phenomena have been observed on other planets (Mars, Saturn, Jupiter) and even on the Sun (solar tornadoes), their existence in Earth's upper atmosphere remained unconfirmed. Previous studies have noted auroral spirals and high-latitude dayside auroral (HiLDA) spots, but these lacked the intensity and characteristics of a typical hurricane. A hurricane in Earth's upper atmosphere would signify a significant transfer of solar wind/magnetosphere energy and momentum into the ionosphere, typically driven by magnetic reconnection (for southward IMF) or Kelvin-Helmholtz instability (for northward IMF with high solar wind density and speed). This study investigates the possibility of such a phenomenon under extremely quiet geomagnetic conditions, characterized by a long period of strongly northward interplanetary magnetic field (IMF) with very low solar wind density and speed. The research question focuses on identifying and characterizing a space hurricane in the Earth’s upper atmosphere under these unusual conditions and explaining its formation mechanism.

The authors reviewed existing literature on terrestrial hurricanes, space hurricanes observed on other planets, and auroral phenomena in Earth's ionosphere. They discussed the known mechanisms for coupling solar wind energy into the magnetosphere-ionosphere system, primarily magnetic reconnection (for southward IMF) and Kelvin-Helmholtz instability (for northward IMF with high solar wind parameters). The literature established that energy transfer is typically weak under extremely quiet geomagnetic conditions. Prior studies on high-latitude dayside aurora (HiLDA) spots, while showing localized activity during northward IMF, lacked the scale and characteristics of a hurricane. This sets the stage for the current study's novel observation and analysis of a space hurricane under unexpectedly quiet conditions, advancing the understanding of magnetosphere-ionosphere coupling mechanisms beyond previously documented cases.

The study used data from multiple sources to observe and analyze a space hurricane event on August 20, 2014. Interplanetary conditions were monitored using data from the THEMIS B satellite, including IMF components (Bz, By, Bx), solar wind speed and density, and dynamic pressure. Geomagnetic indices (SYM-H, AE) confirmed the quiet geomagnetic activity during the event. Auroral observations were obtained from the DMSP F16 satellite's SSUSI instrument, providing images of the cyclone-like auroral spot. The DMSP F16's SSIES instrument measured ionospheric plasma drift, revealing the circular flow with shears and near-zero flow at the center. The AMPERE system provided global field-aligned current (FAC) maps, confirming the upward FAC associated with the hurricane. Further DMSP F16 data from the SSJ4 instrument measured electron and ion precipitation, revealing enhanced electron energy flux and average electron energy within the hurricane. A high-resolution 3-D global magnetohydrodynamics (MHD) simulation using the PPMLR-MHD code, with measured interplanetary conditions as input, was performed to model the formation of the space hurricane and investigate the underlying physical processes. The simulation code utilizes a piecewise parabolic method with a Lagrangian remap to accurately capture MHD shocks and discontinuities. The model incorporates an electrostatic ionosphere shell for coupling between the ionosphere and the magnetospheric inner boundary.

The key findings are: 1. **Observation of a space hurricane:** A long-lasting (8 hours), large-scale (diameter > 1000 km) cyclone-shaped auroral spot with a near-zero flow center and strong circular horizontal plasma flow with shears was observed in the northern polar ionosphere. The event occurred during a period of northward IMF with low solar wind speed and density. 2. **Electron acceleration:** Near the hurricane's center, precipitating electrons were substantially accelerated to ~10 keV, indicating significant energy deposition into the ionosphere. The average electron energy flux was significantly higher than that during substorms or even typical quiet conditions, reaching values comparable to those observed during superstorms. 3. **Upward FACs:** The space hurricane was associated with strong, spot-like upward field-aligned currents (FACs) reaching above 1.5 µAm−2, observed by both DMSP and AMPERE. These FACs were surrounded and closed by downward cusp FACs, maintaining current continuity in the ionosphere. 4. **Simulation Results:** The 3-D MHD simulation reproduced the main observational features of the space hurricane, including the upward FAC funnel, consistent with the observations. The simulation suggested that the space hurricane was generated by steady high-latitude lobe magnetic reconnection, transporting energy and momentum from the magnetosphere to the ionosphere. 5. **Energy and Momentum Transfer:** The space hurricane led to significant energy and momentum deposition into the polar ionosphere despite the extremely quiet geomagnetic conditions. The event's intensity was significantly higher than typical quiet conditions, highlighting the need for improved space weather indicators to account for these polar cap events.

The findings address the research question by demonstrating the existence of space hurricanes in the Earth's upper atmosphere under unexpectedly quiet geomagnetic conditions. The formation mechanism, as revealed by both observations and simulations, involves steady high-latitude lobe magnetic reconnection during northward IMF with low solar wind parameters. This differs from the drivers of tropospheric hurricanes, which rely on strong convection from below. The space hurricane represents a highly efficient channel for energy and momentum transfer from the magnetosphere to the ionosphere. The significantly enhanced energy flux in the space hurricane compared to normal quiet and even superstorm times suggests that current geomagnetic activity indicators may not fully capture the dynamic processes occurring in the polar cap. The significant energy deposition associated with space hurricanes could have implications for space weather, potentially affecting satellite drag, HF radio communications, and various radar and navigation systems. The identified mechanism may also apply to other magnetized bodies in the universe, suggesting space hurricanes are a potentially universal phenomenon.

This study provides the first observational evidence and simulation support for a space hurricane in Earth's polar ionosphere, occurring under surprisingly quiet geomagnetic conditions. The findings significantly advance our understanding of magnetosphere-ionosphere coupling and highlight the importance of high-latitude lobe reconnection in energy transfer during northward IMF. Future research could focus on statistical studies to determine the frequency and occurrence rates of such events and further investigate their implications for space weather forecasting and effects on technological systems. The universality of this phenomenon across other magnetized celestial bodies warrants further exploration.

The study is based on a single observed event. While the simulation results support the observations, a larger dataset would strengthen the conclusions about the generalizability of the findings. The modeling relied on simplified assumptions about the ionosphere and magnetosphere. More sophisticated models incorporating more detailed physics may provide further insight into the complexities of space hurricane dynamics. The limited spatial resolution of some instruments may have influenced some aspects of the observations.