Kp, Dst and Bz: what is the difference?

Three numbers, three questions

Before comparing a number with another chart, ask where it was measured, what its units mean and which time interval it represents. A recent upstream observation and a completed ground-based index describe different information, even if they appear next to each other on a dashboard.

Three measurements

Different places.
Different questions.

BzUpstream of Earth

Which way does the field point?

Unit

nT

Time window

Product-dependent cadence

A component of the solar-wind magnetic field. In GSM coordinates, negative means southward.

A possible driver; read its duration and the other solar-wind measurements.

KpGround observatory network

How disturbed is the field?

Unit

0–9 index

Time window

3-hour intervals

A planetary measure built from standardized magnetic disturbance indices at 13 stations.

A response measure; the scale is not linear and is not measured in nT.

DstLow-latitude ground stations

How has the horizontal field changed?

Unit

nT

Time window

1-hour intervals

An index of large-scale magnetospheric current effects, derived from four stations.

Another response measure; ring, tail and magnetopause currents contribute.

The distinctions above follow the ISGI Kp definition ^[1], ISGI Dst definition ^[2] and NOAA solar-wind product ^[4].

Kp: how disturbed is Earth’s magnetic field?

Kp combines standardized disturbance indices from 13 ground observatories into a planetary measure. Its usual scale runs from 0 to 9, with intermediate third-step values, and each index covers three hours. The scale is quasi-logarithmic: Kp 6 does not mean twice the physical disturbance of Kp 3. ISGI’s Kp documentation ^[1].

NOAA’s geomagnetic G scale is based on Kp. It is a storm-classification system, not a measurement of how a particular person feels or whether aurora will be visible from a particular garden. NOAA’s geomagnetic-storm overview ^[5].

Our Kp guide explains the scale in detail. When checking space weather today, distinguish the running estimate from a completed three-hour interval and read the timestamp attached to each.

Dst: what changed in the low-latitude field?

Dst stands for disturbance storm time. It is expressed in nanotesla (nT) and is derived from hourly observations at four low-latitude stations, after background and regular daily variations are removed. It tracks the magnetic signature of large-scale magnetospheric currents, including ring, tail and magnetopause currents. Calling it a pure ring-current reading would be too simple. ISGI’s Dst documentation ^[2].

During the main phase of many storms, Dst becomes more negative as the low-latitude horizontal field is depressed. But Dst can also be positive: increased solar-wind pressure can compress the magnetosphere and produce a positive disturbance. The sign describes a field change, not a good/bad label. Kyoto’s Dst interpretation ^[3].

This article explains Dst; the Schumann Pulse current-data page does not provide a live Dst feed.

Bz: which way does the upstream field point?

Solar wind carries a magnetic field. Bz is one component, not the total field strength. In the GSM coordinate system used by NOAA’s product, negative Bz means a southward component. The product includes upstream spacecraft measurements, typically near the Sun–Earth L1 region; its source and displayed averaging can change. Check the spacecraft, coordinates and time resolution. NOAA’s solar-wind display documentation ^[4].

A sustained southward field can favour energy transfer into Earth’s magnetosphere. Duration and the surrounding solar-wind conditions matter: a brief negative reading is not enough to assign a storm level. NOAA’s storm-driver explanation ^[5]. Our solar-wind guide adds speed, density and total field strength to that picture.

Why can Bz be negative while Kp is still low?

First check whether the readings describe the same period. An upstream sample is not automatically a measurement of what is happening at Earth at that instant. Then check persistence: a short southward excursion and a long southward interval are different situations. Finally, remember that Kp summarises a three-hour ground response.

These measurement differences explain why the displays need not move together. They do not let you diagnose a particular event from two isolated numbers. For an actual event, inspect the full dated records and the relevant forecast or alert.

Can I convert Bz or Dst into Kp?

There is no universal direct conversion. Bz and Dst share the unit nT, but refer to different field quantities and locations. Kp uses a different scale and combines ground disturbances over a different interval. Adding the three values or putting them on one unlabelled vertical scale would erase those distinctions.

A useful reading sequence is: source and time → upstream conditions → observed ground response → local viewing conditions. If your question is whether you can see the northern lights, continue with the aurora visibility guide; no single number completes that answer.

Sources

References in the text lead here. Original publications open in a new tab; this guide stays open.

• [1] ISGI: Kp index, network, scale and time resolution (opens in a new tab) First mention ↑.
• [2] ISGI: Dst index, derivation and current systems (opens in a new tab) First mention ↑.
• [3] World Data Center for Geomagnetism, Kyoto: Dst interpretation, section 3 (opens in a new tab) Back to text ↑.
• [4] NOAA SWPC: solar-wind measurements, GSM display and source metadata (opens in a new tab) First mention ↑.
• [5] NOAA SWPC: geomagnetic storms, drivers and indices (opens in a new tab) First mention ↑.

Your next observation

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