The aurora forecast is updated for September 23. Keep an eye out for our daily weather forecast update below and tour status as noted on the Anchorage Aurora Quest page. The Kp estimate is for the maximum level forecasted for the time between 11:00 p.m. and 4:00 a.m. Alaska time.
THU SEP 23 – Planetary K-Index [predicted]: Kp 2
FRI SEP 24 – Planetary K-Index [predicted]: Kp 3
SAT SEP 25 – Planetary K-Index [predicted]: Kp 3
Additional Information Affecting Aurora Viewing
Moon: The moon will rise at 8:41 p.m. as a 90.14% waning crescent and will be up all night. The moon will interfere with quiet aurora while it is up in the sky.
Twilight: Nautical twilight ends at 9:29 p.m. and will resume at 6:14 a.m. (Sept. 24). Twilight will not be a factor in aurora viewing.
Solar Winds: Slight enhancements in the Interplanetary Magnetic Field are expected to continue over the course of 23 Sep. Conditions should trend nearer to background levels on 24 Sep before a negative polarity Coronal Hole High Speed Stream (CH HSS) arrives on 25 Sep, elevating conditions once again.
Geomagnetic Field: Quiet to unsettled conditions, with a chance for an isolated active period, are expected 23 Sep. Quiet to unsettled conditions are expected 24 Sep as residual glancing influence from the 19 Sep CME and positive polarity CH HSS wanes. A negative polarity CH HSS is anticipated to affect Earth on 25 Sep and increase the chances for geomagnetic storming. A max Kp of 4 is anticipated.
Planetary K-Index provides an estimate as to how wide the auroral oval may extend. It does not indicate whether the aurora borealis will appear. The most important factors for aurora viewing are the solar winds and geomagnetic field – so note the information above on those. The Planetary K-Index is measured on a scale of 0-9. Anchorage is within the Kp3 zone (aurora visible overhead), but auroras are often seen here on the horizon as low as Kp1 (except for the early and late season, where a higher Kp is needed.) A fairly accurate Kp index can be determined up to 3-days in advance. For the Kp index indicated in the Anchorage northern lights forecast, that is the maximum Kp predicted during the prime aurora viewing time in the Anchorage area (generally between 9:00 p.m. and 4:00 a.m.). We do not provide the Kp level for a forecast period outside of nominal viewing times (i.e., local daytime).
Last Night’s Aurora
- The maximum level for the Planetary K-Index was a Kp4 at prime viewing time for Alaska.
- The maximum hemispheric power achieved last night was 33.1 at 11:44 p.m.
- The IMF was slightly disturbed due to a weak positive polarity CH HSS and the likely arrival of a glancing blow from the 19 Sep CME late in the period. Total field was 3-6 nT, Bz ranged 4 to -5 nT and wind speeds were ~360-405 km/s. Phi was predominantly positive, but began to oscillate to the negative solar sector at approximately 23/0238 UTC.
- The geomagnetic field was quiet to active.
Current Aurora Ovation
Hemispheric Power is a measure of auroral activity that helps determine how bright and active auroras might be. It measures the rate of deposition of charged particles (mainly electrons and protons) into the atmosphere, where they collide with upper atmosphere particles and eventually stop. This process transfers their kinetic energy to the upper atmosphere. The higher the number, the more charged particles are depositing in the upper atmosphere. It’s measured on a scale of 5-150 GW [Gigawatts]. A power level of 20 or more is usually adequate to produce auroras visible to the naked eye. Hemispheric Power fluctuates and is a short range forecast that can be determined only up to around 30-90 minutes in advance with relative accuracy.