When the Sun Strikes: G4 Geomagnetic Storm Explained + Preparation

The second most intense storm on the space weather scale of NOAA is a G4 geomagnetic storm. It occurs when the coronal mass ejections (CMEs) of the Sun cause the charged particles to hit the Earth, disrupting its magnetic field. It's well known that these storms can disrupt power grids, satellites, GPS systems, and radio communications.

In this post, we'll explore the causes of G4 storms, the science behind these storms, and their impact on Earth. We will also provide you with the way you can observe the auroras safely and what actions can be taken to gear up against the dangers caused by these storms.

Just recently, a massive and prolonged G4 storm hit Earth on November 11-14, 2025, causing radio blackouts in Africa and Europe, and widespread auroras visible as far south as Arizona, Texas, Florida, and parts of Mexico.

Why It's Called a G4 Storm, and the KP Index

Geomagnetic storms are classified in grades of five, from G1 (Minor) to G5 (Extreme). This NOAA system quickly shows how strong a storm is and its likely effects on technology and infrastructure. Here's a quick look at all 5 categories:

1. G1 (Minor): High latitudes (e.g., Michigan, Maine) will have slight grid fluctuations, minor satellite impacts, and auroras. A G1 storm occurs 1700 times in an 11-year solar cycle.
2. G2 (Moderate): There is a possibility of voltage alarms in high latitudes; the spacecraft might require orientation corrections, and the auroras can be seen as far south as New York and Idaho. A G2 storm occurs about 600 times per solar cycle.
3. G3 (Strong): Voltage corrections are needed, and it triggers false alarms on power systems. There is also additional drag on the satellites, and intermittent navigation can happen. In addition to this, auroras are visible as far south as Illinois and Oregon. A G3 storm occurs around 175 times in one solar cycle.
4. G4 (Severe): Widespread voltage issues, some grid protective trips, satellites experience surface charging and tracking problems, and HF radio blackouts. Also, auroras can reach Alabama and northern California. A G4 storm occurs about 100 times per solar cycle.
5. G5 (Extreme): Potential widespread grid collapse or blackouts, transformer damage, severe satellite charging and orientation issues, and HF radio blackouts across the sunlit side of Earth. Auroras can be visible at low latitudes, down to Florida and southern Texas. A G5 storm occurs about 4 times per solar cycle.

The Kp Index

The Kp index (Planetary K-index) is a global measure of geomagnetic activity, showing how much the Sun's solar wind disturbs Earth's magnetic field. It is the main metric used to define and forecast geomagnetic storms and corresponds directly to the G1–G5 storm scale. For example, a G4 storm occurs when the Kp index reaches 8. The Kp index is computed at three-hour intervals based on the records of 13 magnetometer stations located all over the world. Most of them are in sub-auroral latitudes.

In order to reach one number, local magnetic disturbances are measured at each station and then averaged. The scale is from 0 to 9, which is further classified into 28 values (0, 0+, 1-, 1, 1+, 2-, 2, etc.). Greater numbers represent more severe geomagnetic disturbances. This system assists power grid industries, satellites, aviation, and pipelines in preparing and taking protective measures when storms occur.

The Causes and the Science Behind G4 Storms

A G4 storm usually begins when the Sun releases a massive burst of plasma and magnetic fields called a coronal mass ejection (CME). The CMEs may travel at a few million miles per hour and are caused by a sudden discharge of magnetic energy in the atmosphere of the Sun. Most of the time, the energy is accompanied by a burst of a solar flare. When the CME is directed towards Earth and has a southward magnetic direction, it can effectively interact with the magnetic field.

By the time the CME hits the Earth, it compresses the magnetosphere, and its energy is transferred by way of what is known as magnetic reconnection. This generates strong electric currents that can cause currents on power lines and pipelines, and heat and expand the upper atmosphere. This adds drag to satellites in low-Earth orbit and interferes with radio and GPS signals.

At the G4 level, there might be voltage issues within the power systems. Satellites may be subject to charging and tracking errors, and high-frequency radio communication may be unreliable for hours. But at the same time, bright auroras may be seen as far south as Alabama or northern California.

Effects of G4 Storms on Earth

A G4 storm has two important impacts: spectacular lights in nature, such as the appearance of an aurora, and the breakdown of modern technology. The charged particles released by CMEs create light. Green and red when they collide with oxygen, and blue and purple when they do the same with nitrogen. In a G4 storm, this activity is amazing, such that the auroras may extend a long distance to the south, as far as Texas, Florida, and Alabama.

At the same time, the storm's energy can affect technology. Power lines may have strong electrical currents flowing through them, leading to voltage problems or transformer stress. The instability of the ionosphere also interferes with high-frequency radio communications and decreases the precision of GPS. Whereas satellites can face the danger of high drag and radiation damage.

The November 2025 G4 storm clearly showed these effects. It caused extensive low-latitude auroras in Arizona, Texas, Florida, Canada, and some parts of Europe, caused by a series of CMEs and an X5.1-class solar flare. However, airlines, power grids, and satellite operators had taken some preventive measures following early warnings issued by the Space Weather Prediction Center (an agency of the NOAA). This restricted the consequences to radio interruptions and small-scale GPS malfunctions. But no significant grid breakdowns were reported.

To mitigate these risks, having backup power is key. For instance, the BLUETTI Elite 400 (3,840 Wh, 2,600 W) can power essentials like refrigerators, phones, and medical devices during outages.

When Is the Best Time to View the Auroras?

The experience of observing auroras in a G4 geomagnetic storm is indeed an incredible one, but it is not only contingent upon geomagnetic storms. So, here are the variables you must remember:

The Prerequisites

In order to view an aurora, a powerful G4 geomagnetic storm or one with a Kp index of 8 or above is required, which can shift the auroral zone southwards. Without this level of activity, viewing is unlikely. Even when there is a storm, the visibility varies depending on the local conditions, such as

1. Darkness: You must have very dark skies away from the city lights.
2. Visibility: The sky should be absolutely clear, as clouds and light pollution are capable of blocking the aurora relatively easily.

Your Action Plan

Now, as the G4 storm is active and if the forecast is clear, here's what you can do next:

1. Go north if possible: During a strong storm, each degree of latitude helps. This can entail commuting to the north in the southern U.S. for several hours.
2. Locate a dark place: Find a Dark Sky site, a state park, or a rural area using a light pollution map. Auroras are brighter and clearer when the sky is dark.
3. Get your eyes used to it: Don't use your phone and keep the bright lights down for 20 minutes to allow your eyes to spot small auroras.
4. Look at the north: Auroras usually begin as a light green line or curve along the northern horizon and then run across the sky.

Tools and Information Required

Next, you'll need info and some apps for updates on aurora activity and visibility:

1. Aurora apps: Applications such as My Aurora Forecast and Alerts and Aurora Alerts indicate the current Kp index and give a warning when geomagnetic storm activity is growing.
2. Official predictions: The 30-minute aurora forecast on the NOAA Space Weather Prediction Center offers dependable short-term forecasts.
3. Location surveillance: Before going out, one should always observe the local cloud cover and use the internet to view aurora webcams north of your location so as to ascertain they are visible.

Pro Tip: Smartphone cameras can be highly sensitive to the colors of the aurora, even more than your eyes. A slight gray or green luminance might appear bright green and purple in an image. So, use a tripod or a stable surface to hold your phone steady to achieve optimal results.

Risks of G4 Storms and How to Prepare for Them?

While the G4 storms do not pose any serious grid disruptions, they highlight that our power infrastructure is susceptible to outages and communication disruptions. Thus, emergency backup power is very important. Beyond power solutions, prepare an emergency kit with water, non-perishable food, flashlights, and first-aid supplies.

Here's a comparison of two great options:

A smaller but high-capacity backup like the BLUETTI Elite 400 features a 3,840 Wh battery and a 2,600 W power output. It is able to power such appliances as refrigerators, smartphones, laptops, radios, GPS, LED lights, and WiFi for days in case the G4 storm overloads the grid.

The unit also has wheels and an extendable handle, and you can take it anywhere you want. Furthermore, it can be charged to 80% using AC power in 1.9 hours, and with dual AC + solar input, you can fully charge it in 70 minutes. But with a solar-only input, it can hit 100% in 6 hours. The Elite 400 features a 15 ms UPS to consistently keep the appliances functional without disruption. It also offers an ultra-low 3W standby mode that allows you to run your low-watt devices for a long time.

Another option, the BLUETTI Apex 300, offers automatic, whole-residence backup with a 2764.8 Wh capacity and a 3840 W output. It is able to offer backup for crucial equipment such as communication and medical apparatus, and even provides 0 ms switchover. But in case you need to have power-intensive appliances running at the same time as your crucial appliances during extended power outages, you can upgrade the unit to 11.5 kW and 58 kWh.

In addition to this, the Apex 300 has fast AC, solar, dual AC and solar, and generator charging. It is capable of reaching 80% in 40 minutes using solar, 100% in 65 minutes using AC and solar, and a generator. With AC, it can get up to 80% in 45 minutes and 30 kW with SolarX 4k & AT1. In addition, with a 12 kW bypass, Apex 300 can support heavy loads like HVAC systems and EVs. It also provides early storm alerts with the BLUETTI app, and can automatically prepare for them.

FAQs

1. Is a G4 storm dangerous to people on the ground?

No. The atmosphere of the planet shields us against direct radiation. Therefore, the greatest threats of a geomagnetic storm are to technology and infrastructure rather than to human health.

2. How much warning do we get before a G4 storm?

Large CMEs can be spotted when they leave the Sun, giving 1–3 days of warning. Their exact strength and arrival time become clearer in the hours before impact.

3. What should I do to prepare for a G4 storm?

The way to get ready in case of a G4 storm is similar to a blackout. Store backup batteries and chargers, and have an emergency kit available. Make certain to disconnect sensitive electronics in case there is a severe storm warning.

4. How often do G4 storms occur?

G4-level storms occur quite often. Around 100 storms happen every 11-year solar cycle. Their average occurrence is about 8 storms/year during the solar maximum, the peak of the solar activity.

Conclusion

A G4 geomagnetic storm is a strong space weather phenomenon that is capable of generating beautiful auroras seen way to the south. But it can also result in voltage, satellite, radio, or GPS problems. To have backup power for communication and other crucial devices, it is important to have a dependable system.

The BLUETTI Elite 400 is a high-capacity portable power station, which allows important units such as refrigerators, WiFi, and medical equipment to operate for many days. Whereas the BLUETTI Apex 300 provides a whole-residence backup, rapid recharging, high-load capability, and storm notifications for keeping your home powered during blackouts. You can also scale it up to operate power-hungry appliances.