Draft:Polar Cap Absorption events

Polar Cap Absorption (PCA) events are primarily caused by high-energy protons of solar origin. PCA events and the associated high frequency (HF) radio blackout pose unique problems to commercial and military aviation. Routes that transit polar regions, especially above about 82-degrees north latitude, can only rely on HF radio communications. Hence, if PCA events are ongoing or forecast, commercial airlines are required to redirect their routes such that HF communications remain viable.

The polar cap region is defined by the open/closed boundary (OCB) of the geomagnetic field. Solar energetic protons (SEP) ranging in energy from 1-200 MeV have easy access to earth's ionosphere near the geomagnetic poles and are responsible for the majority of ionospheric absorption of high frequency radio wave energy. The proton energy range most responsible for absorption is about 1-20 MeV.

The Open-Closed Boundary: Definitions

The open-closed boundary (OCB) defines the region, moving pole-ward, where geomagnetic field lines transition from being closed to open. Closed field lines have both foot points at or near Earth's internal magnetic field in opposing hemispheres. Open field lines have one foot point at Earth while the other maps to the interplanetary magnetic field (IMF) and to the solar wind. Charged particles are able to follow these open field lines into Earth's upper atmosphere.

The OCB defines the polar cap boundary (PCB). Being able to identify and track the OCB allows study of several important dynamic process in Earth's geomagnetic system. Variations in the OCB and related changes in the size of the PCB have been linked to the net rate of magnetic reconnection on both the dayside and nightside.

The OCB is critical to other topics in space physics including planetary magnetospheres and magnetosphere-ionosphere (M-I) coupling. The OCB could also provide important insights into the equator-ward limits of precipitation of energetic particles into the D-region of the ionosphere, causing polar cap absorption (PCA) events.

OCB: Location

The OCB is estimated using in situ measurements made by spacecraft transiting polar regions or by ground-based optical imagers to develop proxies for mapping the OCB. In addition, physics-based computational models can be used to estimate the location of the OCB.

A difficulty with this approach is two-fold. First, spacecraft observations are limited by their single-point sampling at the OCB. Second, ground-based imagers typically have a limited field of view. A significant barrier to a global determination of the OCB lies in the necessity to combine diverse data sets and a lack of true global coverage of observational data.

OCB: The Polar Cusp

A specific region of the dayside OCB is called the cusp, polar cusp, or separatrix. The cusp is located near the noon meridian plane and is an area where geomagnetic field lines provide direct connection between the ionosphere and the solar wind via open magnetic field lines. The location of the cusp depends on several factors including dynamic pressure of the solar wind, IMF orientation, and dipole tilt angle. Generally, the cusp is thought to be at 77 degrees geomagnetic latitude.

PCA Events: Processes

SEPs are emitted from the sun following a solar flare, or more likely a coronal mass ejection (CME). The SEPs, being positively charged, follow the field lines of the interplanetary magnetic field (IMF) toward earth. Once near earth, the SEPs interact with the geomagnetic field, where some are able to enter the cusp region via open field lines. SEPs of higher energy can enter earth's upper atmosphere along closed field lines near the OCB.

PCA Events: Properties

PCA events occur on average about 6 times per year, though there can be well over 10 during an active year and none during quiet times. Once a PCA event begins it will tend to last for about 1-4 days on average, though some have lasted more than a week. The initial increase in absorption begins very near earth's magnetic pole, then over a period of several hours the increased absorption spreads equator-ward covering the polar cap region (as defined by the OCB) with higher energy SEPs reaching further equator-ward.

The initial distribution of SEPs is not homogeneous, so absorption due to PCA events is initially not uniform throughout the region. Over time as the PCA event matures the effects expand to fill the polar cap. As the event dissipates over time the non-uniform characteristic returns until the event concludes.

PCA events are a topic of continued scientific interest. Understanding the nature of PCA events allows those that rely on HF communications to prepare for and adapt to communications disruptions due to PCA events. Understanding the mechanisms that cause PCA events allows space weather forecasters to assist those that rely on HF communication to have advanced warning of PCA events and forewarning of potential consequences.