DISCLAIMER: Harry Potter, names, characters, and related indicia are copyright of Joanne K Rowling (1997). I am dreaming in her world and making no money from it.
AUTHOR'S NOTE: I wish that I could thank Chary, my beta-reader, enough for picking through this bit of fanfic with a fine-tooth comb. I have read thousands of beta-reader thank-yous; and, before Chary, never really understood the amount of effort, work, and heart that they must put into each story. To say that Chary made this better is not clichéd, it is the truth.
This fanfic is consistent with Book 5 (OotP) canon.
Harry Potter and the Letter from Mafalda Hopkirk
by Miss Stephanie D. Curtin
Dear Mr Potter,
With reference to your owl of the fourteenth of July, we are very pleased with the interest that you have shown in this office, and more than happy to answer your questions.
To put it in basic terms, the Improper Use of Magic Office (IUMO) has a map that shows magical density relative to wizarding population. We have four indicator levels classified: Muggle, which shows no magic at all; Low, which covers wizarding children and only a few magical artifacts; Ambient, which covers between one and three wizards, some wards or latent spells, and a basic number of magical artifacts; and Focused, which is the level that spikes when a spell is cast. Usually, Low levels show Muggle-borns, Ambient levels show small or mixed wizarding families, and Focused levels are only permanent in such wizarding places as Diagon Alley, Hogsmeade, and Quidditch pitches. Therefore, when the indicator briefly spikes to Focused in a Low-level or Muggle-level area, we are alerted to a possible infraction.
In your case, number four, Privet Drive, has a constant Ambient level. Due to the current situation and, yes, to special information that we have received from Minister Fudge, we are not alarmed by the fact that your area is now in the topmost Ambient range and often leaks into the lower Focused levels for hours at a time. Please do not hesitate to contact me by owl if you wish the IUMO to send over Magical Law Enforcement (MLES) whenever the magic in your area enters the Focused range.
As for how we can identify spells, when the map indicator shows a spike into the Focused range of magic, we analyze the signature for clues to which spell was cast. As you know from your lessons at Hogwarts, each category of spell has a different signature and level of necessary magic. From these two sources of information, we can almost always deduce which spell was cast.
To tell who has cast the spell, we cross-reference the type and level of spell with the wizard(s) registered in that area. Because most improper use of magic happens around Muggles, it is easy to identify the caster. In the case of a group participating in Muggle-baiting, additional investigation by the MLES and analysis of their wands is necessary.
In most cases, all that is required by this office is the occasional letter of warning for underage wizards and witches living in Muggle areas. However, we also have a mandate to alert the Obliviators or the Accidental Magic Reversal Squad or St Mungo's Hospital should the case warrant it. We also aid the MLES and the Aurors with long-term surveillance projects.
You are right in thinking that there are ways to mask or otherwise befuddle the IUMO's surveillance methods. That information, however, is restricted to members of this office and to the Aurors. Because the system is not perfect, it is well-known within the Ministry that the IUMO should only be used as supplementary evidence in any case of improper use of magic and not as evidence to convict.
If you have any further questions about the workings of the Improper Use of Magic Office, please do not hesitate to contact me by owl. I am also available between 8.30 a.m. and 5.30 p.m., Monday to Friday, if you wish to make an appointment to visit the IUMO proper at the Ministry of Magic.
With best wishes,
Improper Use of Magic Office
Ministry of Magic