Every aurora forecast that goes beyond “check the Kp index” eventually points you at a live solar wind plot — a set of scrolling charts showing measurements taken by a spacecraft sitting far upstream of Earth. It looks intimidating at first, but it comes down to four values, and once you know which one matters most, the rest falls into place quickly. For the live chart itself, see this site’s solar wind page; this guide is the reference explanation of what each line on it actually means.
Where the data comes from
Solar wind monitoring spacecraft sit near the L1 Lagrange point, a gravitationally stable spot about 1.5 million kilometres from Earth in the direction of the Sun — roughly four times further away than the Moon. From there, they measure the solar wind before it reaches Earth, giving forecasters a genuine head start: typically 30 to 90 minutes, depending on the wind’s speed. That head start is also the ceiling on how far in advance a truly reliable aurora forecast can be made — see how aurora forecasts work for why this window, not a longer one, is the real state of the art.
Two genuinely separate instrument systems, riding on the same spacecraft, produce the numbers you actually see on a plot. A magnetometer — typically a set of fluxgate coils sensitive enough to detect changes of a fraction of a nanotesla — measures the magnetic field directly, giving Bz, By and Bt. A completely different instrument, a Faraday cup or similar electrostatic analyser, measures the flow of charged particles (mostly protons) striking a charged plate, from which speed, density and temperature are derived. Because these are independent systems, it’s entirely possible for one to report clean data while the other has a gap — worth knowing before assuming a blank stretch on a chart means nothing at all happened.
Bz: the number that matters most
Bz measures the north-south component of the interplanetary magnetic field carried along by the solar wind, in a coordinate system (GSM) oriented relative to Earth’s own field and the Sun-Earth line. Earth’s magnetic field points northward near the equator. When the solar wind’s Bz points southward — shown as a negative number, often by convention plotted so “down” on the chart means southward — it can efficiently connect with Earth’s field through a process called magnetic reconnection, pouring energy into the magnetosphere. This is, more than any other single factor, what actually drives geomagnetic storms and aurora.
A northward (positive) Bz tends to shield Earth’s magnetosphere from the solar wind’s energy, even if that wind is fast and dense. This is why a spacecraft can show a big, fast solar wind stream arriving with seemingly little geomagnetic consequence: if Bz stays positive, the door stays largely shut. Reconnection efficiency also isn’t a simple on/off switch — a Bz of -5 nT connects only modestly, while -15 nT or beyond couples energy into the magnetosphere far more efficiently, which is part of why forecasters watch not just the sign of Bz but how strongly negative it gets and how long it stays there.
By: the quieter sibling
Alongside Bz, most solar wind plots also show By, the east-west component of the same interplanetary magnetic field. By doesn’t drive geomagnetic activity as directly as Bz does, but it isn’t irrelevant: together, Bz and By define what’s called the field’s “clock angle” — its orientation in the plane perpendicular to the Sun-Earth line — and that clock angle helps determine which hemisphere and which local-time sector of the auroral oval is likely to brighten first during a given event. For a casual forecast check, Bz alone is enough; By becomes more useful once you’re trying to understand why one side of the oval lit up before the other on a specific night.
Bt: the total field strength
Bt is the magnitude of the total interplanetary magnetic field, independent of direction — essentially, how strong the magnetic field is overall, combining its north-south, east-west and Sun-Earth-aligned components. A higher Bt means there’s more “magnetic material” available to potentially turn southward; it sets an upper bound on how negative Bz could plausibly get, but it doesn’t tell you the direction on its own. Watching Bt rise ahead of a Bz swing is often an early sign that something significant (frequently a CME’s leading edge, or the compressed boundary of a corotating interaction region) has arrived, even before its direction becomes clear. A Bt reading that’s merely elevated but stays flat and unremarkable for hours is a much weaker signal than one that rises sharply and then starts oscillating — real storms often show Bz swinging rapidly between north and south within an elevated Bt envelope, rather than settling calmly on one value.
Speed: how fast, how soon
Solar wind speed, measured in kilometres per second, ranges from a quiet background of around 300-400 km/s up to 700-800 km/s or more during a fast coronal hole stream (see coronal holes and recurrent storms) or well over 1,000 km/s for an unusually fast CME. Faster wind compresses Earth’s magnetosphere more forcefully and arrives with less warning after leaving the Sun, but speed alone, without a southward Bz, produces a comparatively modest geomagnetic response. Speed also sets your lead time directly: solar wind travelling at 400 km/s takes roughly 100 minutes to cross the 1.5 million km from L1 to Earth, while a 1,000 km/s stream covers the same distance in only about 25 minutes — which is why an unusually fast CME can compress the useful warning window well below the “typical” 30-90 minutes quoted elsewhere on this site.
Density: the multiplier
Proton density, measured in particles per cubic centimetre, describes how much material is actually flowing past — a dense solar wind carries more energy for a given speed than a sparse one. A dense, fast stream with a strongly southward Bz is the combination that produces the most dramatic storms; density alone, with a northward or weak Bz, mostly just makes for a slightly more disturbed but not necessarily aurora-rich few hours. Density and speed together also determine dynamic pressure — roughly proportional to density multiplied by speed squared — which compresses the entire magnetosphere and can, on its own, trigger a brief geomagnetic disturbance (sometimes visible as a sudden impulse in ground magnetometer data) even independent of Bz, though this effect is generally short-lived compared with a sustained southward-Bz-driven storm.
A worked example: reading a specific combination
Numbers make this concrete. Imagine a plot showing Bz at -15 nT, Bt at 18 nT, speed at 650 km/s, and density at 9 particles/cm³, all sustained for the past hour. Here’s how to read that: Bz at -15 nT is solidly in “meaningful driver” territory, well past the rough -10 nT threshold mentioned above. Bt at 18 nT confirms there’s a strong field overall, and with Bz at -15 of an 18 nT total, the field is oriented almost entirely southward rather than partially cancelled by a large By — an efficient geometry for reconnection. Speed at 650 km/s is a genuinely fast stream, well above the quiet 300-400 km/s background, arriving with roughly 40 minutes of remaining lead time from L1. Density at 9 particles/cm³ is moderately elevated. Combined, this reads as a fast, dense, strongly and persistently southward stream — the textbook setup for a strong geomagnetic storm, and exactly the kind of reading that would justify checking Tromsø, Yellowknife or Reykjavík’s live verdict immediately rather than waiting for an updated Kp figure. Compare that with Bz at +8 nT, speed at 700 km/s, density at 12: despite the fast, dense stream, the northward Bz means the door to the magnetosphere is largely shut, and geomagnetic activity would likely stay modest regardless of how impressive the speed and density numbers look.
Putting it together: what to actually watch for
When checking a live solar wind plot, the practical sequence is: look at Bz first — is it meaningfully negative, and has it been that way for a while rather than just spiking briefly? Then check Bt, to see how much room there is for Bz to go more negative still, and roughly what fraction of the total field Bz represents. Then check speed and density as a rough sense of how forceful the overall event is, and how much lead time you realistically have left. A sustained, strongly negative Bz on a fast, dense stream is the textbook setup for a strong geomagnetic storm and a correspondingly strong aurora display — and it’s exactly the combination that, once observed, tends to trigger a rapid jump in the live verdict on this site’s city pages, often faster than the officially reported Kp index catches up. This matters most for destinations already close to their Kp threshold: a city like Fairbanks or Churchill that only needs a modest push can go from “unlikely” to “good” within the time it takes Bz to swing south and hold there.
Why this beats waiting for Kp alone
Kp is derived, after some delay, from ground-based magnetometer measurements of the disturbance the solar wind has already caused. Watching the solar wind plot directly means watching the cause rather than the effect, which is why experienced aurora chasers keep half an eye on Bz through the evening rather than only refreshing a Kp number. It’s also precisely the reason substorms (see substorms explained) can seem to erupt suddenly: the geomagnetic effect of a sustained southward Bz often builds for a while before it releases in a sudden, visible surge. If you only ever check one number on a night with promising conditions, make it Bz — everything else on the plot is context for how much that southward turn actually matters.
Frequently asked questions
Which number should I watch first: Bz, speed, or density?
Bz, by a wide margin. A fast, dense solar wind stream with a northward Bz will often do very little geomagnetically, while a much slower stream with a strongly negative (southward) Bz can drive a real storm. Speed and density matter, but mostly as multipliers on whatever Bz is doing.
What counts as a "strong" negative Bz?
There's no hard cutoff, but as a rough guide, Bz below about -10 nT sustained for an hour or more is generally considered a meaningful driver of geomagnetic activity, and values beyond -20 nT are associated with strong to severe storms. Brief spikes matter less than a sustained southward reading.
Where does this real-time data actually come from?
Mainly from spacecraft stationed near the L1 Lagrange point, about 1.5 million kilometres from Earth toward the Sun — historically ACE and DSCOVR, with newer missions such as IMAP joining over time. Being upstream of Earth gives roughly 30 to 90 minutes of warning before the same solar wind reaches our planet.
Why does the data sometimes show a gap or look stale?
Spacecraft occasionally lose contact, pass through noisy data periods, or experience instrument issues. A stale or missing reading is a real limitation of any live aurora forecast, including this one — always check the data-source timestamp rather than assuming a chart is current.
What instruments actually measure Bz, speed and density?
Two separate instrument types on the same spacecraft: a magnetometer (a set of sensitive fluxgate coils) measures the magnetic field components including Bz and Bt directly, while a Faraday cup or similar plasma instrument measures the flow of charged particles to derive speed, density and temperature. The two systems are independent, which is why a spacecraft can occasionally report good field data with a gap in plasma data, or vice versa.
What is the By component, and does it matter?
By is the east-west component of the interplanetary magnetic field. It doesn't drive geomagnetic activity as directly as Bz, but it does help determine the exact local time and hemisphere where auroral activity concentrates, and it factors into the combined 'clock angle' (the direction of the field in the plane perpendicular to the Sun-Earth line) that researchers use for a fuller picture than Bz alone.
Does a spike in solar wind density by itself mean anything for aurora?
Rarely on its own. A sudden density jump usually marks a shock front or the leading edge of a CME or stream interaction region arriving, and it's often followed within minutes to hours by more geomagnetically important changes in Bz and speed — so a density spike is a useful 'something just arrived, watch closely' signal even before you know what the field is doing.
How is this different from checking a Kp forecast?
Kp reports what already happened to Earth's magnetic field, three hours at a time (or roughly one minute at a time for the estimated value). The solar wind plot shows the cause, measured before it reaches Earth, which is the only way to get the roughly 30-90 minute lead time described in how aurora forecasts work — by the time Kp moves, the geomagnetic response is already underway.