Space Hurricane

Matthew Ginnow, Matt Ginnow

https://www.nature.com/articles/s41467-021-21459-y

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.

Hurricanes often cause loss of life and property through high winds and flooding resulting from the coastal storm surge of the ocean and the torrential rains¹^,². They are characterized by a low-pressure center (hurricane eye), strong winds and flow shears, and a spiral arrangement of towering clouds with heavy rains¹^,³. In space, astronomers have spotted hurricanes on Mars, and Saturn, and Jupiter⁴^,⁵, which are similar to terrestrial hurricanes in the low atmosphere. There are also solar gases swirling in monstrous formations deep within the sun’s atmosphere, called solar tornadoes with widths of several Earth radii (R_E)⁶. However, hurricanes have not been reported in the upper atmosphere of the planets in our heliosphere. Although vortex structures of aurora, called auroral spirals, often appear to evolve from arc-like auroras to a train of two or more spirals of diameter ~50 km in the Earth’s nightside auroral oval (about 65–75° magnetic latitude)⁷^,⁸, they are not unusually intense and do not have similar features of a typical hurricane. In the Earth’s polar cap region (about 75–90° magnetic latitude), high-latitude dayside auroral (HiLDA) spots, but without spiral or hurricane features, have been reported to be caused by precipitating electrons predominantly during northward interplanetary magnetic field (IMF) with a strongly positive IMF By component⁹^,¹⁰^,¹¹^,¹²^,¹³.

A hurricane is clearly associated with strong energy and mass transportation, so a hurricane in Earth’s upper atmosphere must be violent and efficiently transfer solar wind/magnetosphere energy and momentum into the Earth’s ionosphere. It is well known that magnetic reconnection and Kelvin–Helmholtz (K–H) instability are the most important and fundamental processes for coupling solar wind energy into the magnetosphere-ionosphere system and similar coupling occurs in other astrophysical, space, and laboratory plasmas. For a southward IMF (which occurs nearly half of the time), magnetic reconnection occurs at the low-latitude dayside magnetopause¹⁴^,¹⁵^,¹⁶ and it directly brings solar wind energy and plasma into the magnetosphere¹⁷^,¹⁸^,¹⁹^,²⁰. Under a northward IMF condition, magnetic reconnection is limited to a small high latitude region and K–H instability becomes important in bringing solar wind energy and plasma into the magnetosphere when the solar wind density and speed are high²¹^,²²^,²³^,²⁴^,²⁵^,²⁶^,²⁷. It is generally believed that transfer of solar wind energy and plasma into the magnetosphere and ionosphere is very weak when geomagnetic activity is extremely quiet (such as during a long period of strongly northward IMF with very low solar wind density and speed).

Here, we present an observation of a long-lasting, large and energetic space hurricane in the northern polar ionosphere that deposited solar wind/ magnetosphere energy and momentum into the ionosphere during a several hour period of northward IMF and very low solar wind density and speed.

On 20th August 2014, a relatively stable northward IMF condition (IMF Bz > 0 for more than 8 h) occurred with a large duskward component (IMF By ~13 nT), and roughly stable interplanetary conditions with low solar wind speed and density (Fig. 1a–c). The IMF Bx and Bz decreased slowly from 10 to 5 nT over the 8-h period, and the low solar wind speed (around 340 km/s) and number density (around 2 cm⁻³) indicates a very low dynamic pressure of around 0.5 nPa (gray shading in Fig. 1 indicates the interval of interest). These conditions are not favorable for magnetic reconnection at the low-latitude dayside magnetopause¹⁴^,¹⁵^,¹⁶, nor for triggering of the K–H instability between the solar wind and magnetosphere in the magnetospheric flank regions²¹^,²²^,²³^,²⁴^,²⁵, but are suitable for forming high-latitude dayside auroral spots in the polar cap region⁹^,¹⁰^,¹¹^,¹²^,¹³. The symmetric ring current H index (SYM-H) and auroral electrojet AL and AU indices show non-storm and quiet auroral oval geomagnetic activity during the interval of interest (Fig. 1d, e).

Matthew Ginnow, Matt Ginnow