<p>There was widespread media coverage in advance of of a predicted G3 storm on 2021-10-31 due to a CME. For instance, <a href="https://www.npr.org/2021/10/30/1050810514/northern-lights-halloween-2021" rel="nofollow noreferrer">https://www.npr.org/2021/10/30/1050810514/northern-lights-halloween-2021</a>:</p>
<blockquote>
<p>A solar flare erupted on Thursday morning and is expected to reach Earth on Saturday. NOAA's Space Weather prediction center in Boulder, Colo., announced the possibility of increased visibility of the lights on Thursday when it issued a G3 geomagnetic storm watch for the weekend.</p>
</blockquote>
<p>However, this storm never materialized.</p>
<p>The prediction from SWPC directly was as follows (available at <a href="ftp://ftp.swpc.noaa.gov/pub/forecasts/three_day/10310030three_day_forecast.txt" rel="nofollow noreferrer">ftp://ftp.swpc.noaa.gov/pub/forecasts/three_day/10310030three_day_forecast.txt</a>):</p>
<pre><code>:Product: 10310030three_day_forecast.txt
:Issued: 2021 Oct 31 0030 UTC
# Prepared by the U.S. Dept. of Commerce, NOAA, Space Weather Prediction Center
#
A. NOAA Geomagnetic Activity Observation and Forecast

The greatest observed 3 hr Kp over the past 24 hours was 4 (below NOAA
Scale levels).
The greatest expected 3 hr Kp for Oct 31-Nov 02 2021 is 7 (NOAA Scale
G3).

NOAA Kp index breakdown Oct 31-Nov 02 2021

            Oct 31     Nov 01     Nov 02
00-03UT        5 (G1)     4          3
03-06UT        5 (G1)     5 (G1)     3
06-09UT        6 (G2)     4          2
09-12UT        7 (G3)     3          2
12-15UT        6 (G2)     3          1
15-18UT        5 (G1)     2          1
18-21UT        5 (G1)     2          1
21-00UT        4          3          2

Rationale: G1-G3 (Minor-Strong) geomagnetic storm conditions are likely
on 31 Oct due to the anticipated arrival of a CME from 28 Oct. G1
(Minor) storming is likely on 01 Nov as conditions persist.

B. NOAA Solar Radiation Activity Observation and Forecast

Solar radiation, as observed by NOAA GOES-16 over the past 24 hours, was
above S-scale storm level thresholds.

Solar Radiation Storm Forecast for Oct 31-Nov 02 2021

              Oct 31  Nov 01  Nov 02
S1 or greater   99%      5%      5%

Rationale: S1 (Minor) solar radiation storm  conditions are expected to
persist on 31 Oct.

C. NOAA Radio Blackout Activity and Forecast

No radio blackouts were observed over the past 24 hours.

Radio Blackout Forecast for Oct 31-Nov 02 2021

              Oct 31        Nov 01        Nov 02
R1-R2           35%           35%           35%
R3 or greater    5%            5%            5%

Rationale: There is a chance for R1-R2 (Minor-Moderate) radio blackouts
on 31 Oct - 02 Nov due to the flare potential of Regions 2887 and 2891.
</code></pre>
<p>The actual levels over time are shown in this picture, taken from NOAA as well:</p>
<p><a href="https://i.sstatic.net/HIvmy.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HIvmy.gif" alt="Actual levels" /></a></p>
<hr />
<p>My question is, what happened? Did the CME miss the Earth? Or is there another explanation for the large difference between the prediction and reality?</p>

CC BY-SA 4.0
2022-10-14T13:18:56.530Z
A CME erupted on Thursday and was expected to reach Earth on Saturday, not a solar flare.  The latter is an enhancement in x-rays, so those get to Earth in ~8 minutes not 3 days, e.g., see https://astronomy.stackexchange.com/a/16786/13663
honeste_vivere
CC BY-SA 4.0
2022-10-15T01:00:12.870Z
Later in the article: "When the solar flare eruption happened, it resulted in a coronal mass ejection, or a large cloud of solar-charged particles headed toward Earth. These particles are what may amplify the northern lights." So apparently, it was both. Don't blame me for NPR's imprecise terminology. :)
D0SBoots
<coronal-mass-ejection>
What happened to the predicted geomagnetic storm on 2021-10-31?
D0SBoots
# What happened to the predicted geomagnetic storm on 2021-10-31?

(Asked by: username_0 on 2021-11-01)

There was widespread media coverage in advance of of a predicted G3 storm on 2021-10-31 due to a CME. For instance, <https://www.npr.org/2021/10/30/1050810514/northern-lights-halloween-2021>:

> A solar flare erupted on Thursday morning and is expected to reach Earth on Saturday. NOAA's Space Weather prediction center in Boulder, Colo., announced the possibility of increased visibility of the lights on Thursday when it issued a G3 geomagnetic storm watch for the weekend.

However, this storm never materialized.

The prediction from SWPC directly was as follows (available at <ftp://ftp.swpc.noaa.gov/pub/forecasts/three_day/10310030three_day_forecast.txt>):

```
:Product: 10310030three_day_forecast.txt
:Issued: 2021 Oct 31 0030 UTC
# Prepared by the U.S. Dept. of Commerce, NOAA, Space Weather Prediction Center
#
A. NOAA Geomagnetic Activity Observation and Forecast

The greatest observed 3 hr Kp over the past 24 hours was 4 (below NOAA
Scale levels).
The greatest expected 3 hr Kp for Oct 31-Nov 02 2021 is 7 (NOAA Scale
G3).

NOAA Kp index breakdown Oct 31-Nov 02 2021

            Oct 31     Nov 01     Nov 02
00-03UT        5 (G1)     4          3
03-06UT        5 (G1)     5 (G1)     3
06-09UT        6 (G2)     4          2
09-12UT        7 (G3)     3          2
12-15UT        6 (G2)     3          1
15-18UT        5 (G1)     2          1
18-21UT        5 (G1)     2          1
21-00UT        4          3          2

Rationale: G1-G3 (Minor-Strong) geomagnetic storm conditions are likely
on 31 Oct due to the anticipated arrival of a CME from 28 Oct. G1
(Minor) storming is likely on 01 Nov as conditions persist.

B. NOAA Solar Radiation Activity Observation and Forecast

Solar radiation, as observed by NOAA GOES-16 over the past 24 hours, was
above S-scale storm level thresholds.

Solar Radiation Storm Forecast for Oct 31-Nov 02 2021

              Oct 31  Nov 01  Nov 02
S1 or greater   99%      5%      5%

Rationale: S1 (Minor) solar radiation storm  conditions are expected to
persist on 31 Oct.

C. NOAA Radio Blackout Activity and Forecast

No radio blackouts were observed over the past 24 hours.

Radio Blackout Forecast for Oct 31-Nov 02 2021

              Oct 31        Nov 01        Nov 02
R1-R2           35%           35%           35%
R3 or greater    5%            5%            5%

Rationale: There is a chance for R1-R2 (Minor-Moderate) radio blackouts
on 31 Oct - 02 Nov due to the flare potential of Regions 2887 and 2891.

```

The actual levels over time are shown in this picture, taken from NOAA as well:

[![Actual levels](https://i.sstatic.net/HIvmy.gif)](https://i.sstatic.net/HIvmy.gif)

---

My question is, what happened? Did the CME miss the Earth? Or is there another explanation for the large difference between the prediction and reality?

**Tags:** coronal-mass-ejection

> **Comments:**
> **username_1**: A CME erupted on Thursday and was expected to reach Earth on Saturday, not a solar flare.  The latter is an enhancement in x-rays, so those get to Earth in ~8 minutes not 3 days, e.g., see https://astronomy.stackexchange.com/a/16786/13663  
> **username_0**: Later in the article: "When the solar flare eruption happened, it resulted in a coronal mass ejection, or a large cloud of solar-charged particles headed toward Earth. These particles are what may amplify the northern lights." So apparently, it was both. Don't blame me for NPR's imprecise terminology. :)  

---

## 0 answers

